### 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 tests for the given plugin type and plugin name.
        This function is useful for plugin types which do not require special processing.
        """
        if plugin_name == '__init__':
            return all_tests(self.args, True)
        integration_target = self.integration_targets_by_name.get('%s_%s' % (plugin_type, plugin_name))
        if integration_target:
            integration_name = integration_target.name
        else:
            integration_name = None
        units_path = os.path.join(data_context().content.unit_path, 'plugins', plugin_type, 'test_%s.py' % plugin_name)
        if units_path not in self.units_paths:
            units_path = None
        return dict(
            integration=integration_name,
            units=units_path,
        )
def all_tests(args, force=False):