[lldb] Use the basename of the Python test for the log name instead of the class name

Summary:

From what I know we already have the restriction that every test in the test
suite needs to have a unique file name as that's used for generating the unique
build directory for a test. It seems there is also a restriction that every test
case class in the test suite needs to have a unique name as that's used to
generate the unique log file name for the test run.

This changes the log file format to use the basename of the test file instead so
that we only have to keep worrying about the 'unique file name' restriction from
now on.

This came up because I started naming the test classes "TestCase" (as repeating
the file name in the test class seems like redudant information that just makes
renaming tests a pain).

Reviewers: labath, JDevlieghere

Reviewed By: labath

Subscribers: mgorny, abidh

Differential Revision: https://reviews.llvm.org/D83767