### 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
'units': set(),
        'integration': set(),
        'windows-integration': set(),
        'network-integration': set(),
    }
    focused_commands = collections.defaultdict(set)
    deleted_paths = set()
    original_paths = set()
    additional_paths = set()
    no_integration_paths = set()
    for path in paths:
        if not os.path.exists(path):
            deleted_paths.add(path)
            continue
        original_paths.add(path)
        dependent_paths = mapper.get_dependent_paths(path)
        if not dependent_paths:
            continue
        display.info('Expanded "%s" to %d dependent file(s):' % (path, len(dependent_paths)), verbosity=2)
        for dependent_path in dependent_paths:
            display.info(dependent_path, verbosity=2)
            additional_paths.add(dependent_path)
    additional_paths -= set(paths)  
    if additional_paths:
        display.info('Expanded %d changed file(s) into %d additional dependent file(s).' % (len(paths), len(additional_paths)))
        paths = sorted(set(paths) | additional_paths)
    display.info('Mapping %d changed file(s) to tests.' % len(paths))
    none_count = 0
    for path in paths:
        tests = mapper.classify(path)