### 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
"""Return the classification for the given path."""
        if data_context().content.is_ansible:
            return self._classify_ansible(path)
        if data_context().content.collection:
            return self._classify_collection(path)
        return None
    def _classify_common(self, path):  
        """Return the classification for the given path using rules common to all layouts."""
        dirname = os.path.dirname(path)
        filename = os.path.basename(path)
        name, ext = os.path.splitext(filename)
        minimal = {}
        if os.path.sep not in path:
            if filename in (
                    'azure-pipelines.yml',
                    'shippable.yml',
            ):
                return all_tests(self.args)