[lldb][test] Handle .categories lookup for inline tests.

Summary:
When creating a test with `lldbinline.MakeInlineTest()`, the reported `inspect.getfile(test.__class__)` is `lldbtest.pyc`, meaning any `.categories` file will be ineffective for those tests. Check for the test_filename first, which inline tests will set.

Additionally, raise an error with the starting dir if `.categories` is not found. This makes the problem more obvious when it occurs: when the test is separated from the test framework tree.

Reviewers: labath, JDevlieghere

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D71099