### Summary

ansible-test isn't triggering unit tests for connection or inventory plugins:

```
[✔ mchappel@mchappel aws] $ find tests/unit/plugins/connection/
tests/unit/plugins/connection/
tests/unit/plugins/connection/__init__.py
tests/unit/plugins/connection/test_aws_ssm.py

[✔ mchappel@mchappel aws] $ ansible-test units --color -v --coverage-check --changed --remote-terminate always --remote-stage prod --docker --python 3.7 --base-branch main
Run command: git symbolic-ref --short HEAD
Run command: git for-each-ref refs/heads/ --format '%(refname:strip=2)'
Run command: git merge-base --fork-point main
Run command: git ls-files -z --cached
Run command: git ls-files -z --others --exclude-standard
Run command: git diff --name-only --no-renames -z 111f4308966e8cd0c66f75b787fbb6f75477d595 HEAD
Run command: git diff --name-only --no-renames -z --cached
Run command: git diff --name-only --no-renames -z
Run command: git -c core.quotePath= diff 111f4308966e8cd0c66f75b787fbb6f75477d595
Detected branch units/2021-03-12 forked from main at commit 111f4308966e8cd0c66f75b787fbb6f75477d595
Detected changes in 1 file(s).
plugins/connection/aws_ssm.py
Mapping 1 changed file(s) to tests.
NOTICE: Omitted 1 file(s) that triggered no tests.
WARNING: No tests found for detected changes.
```

```
[✔ mchappel@mchappel aws] $ find tests/unit/plugins/inventory/
tests/unit/plugins/inventory/
tests/unit/plugins/inventory/__init__.py
tests/unit/plugins/inventory/test_aws_ec2.py


[✔ mchappel@mchappel aws] $ ansible-test units --color -v --coverage-check --changed --remote-terminate always --remote-stage prod --docker --python 3.7 --base-branch unit-tests/2021-03-12
Run command: git symbolic-ref --short HEAD
Run command: git for-each-ref refs/heads/ --format '%(refname:strip=2)'
Run command: git merge-base --fork-point unit-tests/2021-03-12
Run command: git ls-files -z --cached
Run command: git ls-files -z --others --exclude-standard
Run command: git diff --name-only --no-renames -z dc70967e80b2cbbc8f6dccf4985e434cd04a9fc1 HEAD
Run command: git diff --name-only --no-renames -z --cached
Run command: git diff --name-only --no-renames -z
Run command: git -c core.quotePath= diff dc70967e80b2cbbc8f6dccf4985e434cd04a9fc1
Detected branch unit-tests/2021-03-12-tmp forked from unit-tests/2021-03-12 at commit dc70967e80b2cbbc8f6dccf4985e434cd04a9fc1
WARNING: Ignored 1 untracked file(s). Use --untracked to include them.
Detected changes in 1 file(s).
plugins/inventory/aws_ec2.py
Mapping 1 changed file(s) to tests.
NOTICE: Omitted 1 file(s) that triggered no tests.
WARNING: No tests found for detected changes.
```

This looks like it's because `test/unit/` (test in the singular) is still hard-coded in `test/lib/ansible_test/_internal/classification.py`

### Issue Type

Bug Report

### Component Name

ansible-test

### Ansible Version

```console (paste below)
$ ansible --version
ansible 2.11.0.dev0 (devel 4c5ce5a1a9) last updated 2021/02/11 13:33:25 (GMT +200)
```


### Configuration

```console (paste below)
$ ansible-config dump --only-changed

```


### OS / Environment

RHEL8

### Steps to Reproduce

- Make a change to a connection or inventory plugin in a collection (which has unit tests)
- Open a PR

### Expected Results

Unit tests should run for the plugin

### Actual Results

Unit tests do not run
"""
        :type path: str
        :rtype: list[str]
        """
        if not self.powershell_module_utils_imports:
            display.info('Analyzing powershell module_utils imports...')
            before = time.time()
            self.powershell_module_utils_imports = get_powershell_module_utils_imports(self.powershell_targets)
            after = time.time()
            display.info('Processed %d powershell module_utils in %d second(s).' % (len(self.powershell_module_utils_imports), after - before))
        name = get_powershell_module_utils_name(path)
        return sorted(self.powershell_module_utils_imports[name])
    def get_csharp_module_utils_usage(self, path):
        """
        :type path: str
        :rtype: list[str]
        """
        if not self.csharp_module_utils_imports:
            display.info('Analyzing C# module_utils imports...')
            before = time.time()
            self.csharp_module_utils_imports = get_csharp_module_utils_imports(self.powershell_targets, self.csharp_targets)
            after = time.time()
            display.info('Processed %d C# module_utils in %d second(s).' % (len(self.csharp_module_utils_imports), after - before))
        name = get_csharp_module_utils_name(path)
        return sorted(self.csharp_module_utils_imports[name])
    def get_integration_target_usage(self, path):