[BUG] Test matrix difficult to grok

[dblock]

What is the bug?

Coming from #823 (comment), there's some code that defines "server tests" which makes it difficult to know what tests actually run in

opensearch-py/test_opensearchpy/run_tests.py

Lines 159 to 164 in a24b9f3

	ignores = [
	"test_opensearchpy/test_server/",
	"test_opensearchpy/test_server_secured/",
	"test_opensearchpy/test_async/test_server/",
	"test_opensearchpy/test_async/test_server_secured/",
	]

.

So what tests actually run and when?

What is the expected behavior?

It should be easy to figure out what runs when. Test files shouldn't be including/excluding anything with nested if's or checking whether we are running with or without plugins.

Refactor the test setup.

1. By default all test are unit tests and should all run without requiring an actual OpenSearch instance.
2. A folder called integ should contain integration tests that can be run in bulk and require an OpenSearch instance.
3. If we want to keep plugin tests separate, then there should be a plugins/<name of plugin> folder to run those.

CI/CD should invoke unit tests by default, and integration tests otherwise across a matrix of versions.