Make python plugin system test agnostic to other managers being installed.

What

Make it so no matter if you have other managers installed into your test environment (ie, BAL,) the plugin test won't fail.

Why

Right now, the plugin test checks against a specific list of strings, assuming you don't have other managers installed. You get this output :

/home/elliotmorris/data/openAssetIO/elliotcmorris/OpenAssetIO/src/openassetio-python/tests/bridge/python/test_hostApi.cpp:39: FAILED:
  CHECK( identifiers == openassetio::Identifiers{ "org.openassetio.test.pluginSystem.resources.modulePlugin"} )
with expansion:
  { "org.openassetio.test.pluginSystem.resources.modulePlugin", "org.
  openassetio.examples.manager.bal" }
  ==
  { "org.openassetio.test.pluginSystem.resources.modulePlugin" }

It's reasonably to have other managers installed in your test environment, especially if you are an external contributor. If you're following our instructions and running the full ctest suite, this may make you think our tests dont work out of the box, and degrade the quality opinion of OpenAssetIO.

Acceptance Criteria

Start in src/openassetio-python/tests/bridge/python/test_hostApi.cpp at the failing identifier CHECK on line 39. Run the affected plugin test with another manager such as BAL installed, then run the full ctest suite. Done means the test passes regardless of additional installed managers while still checking the test plugin identifier.