[lldb][test] Switch LLDB API tests from vendored unittest2 to unittest (#79945)

This removes the dependency LLDB API tests have on
lldb/third_party/Python/module/unittest2, and instead uses the standard
one provided by Python.

This does not actually remove the vendored dep yet, nor update the docs.
I'll do both those once this sticks.

Non-trivial changes to call out:
- expected failures (i.e. "bugnumber") don't have a reason anymore, so
those params were removed
- `assertItemsEqual` is now called `assertCountEqual`
- When a test is marked xfail, our copy of unittest2 considers failures
during teardown to be OK, but modern unittest does not. See
TestThreadLocal.py. (Very likely could be a real bug/leak).
- Our copy of unittest2 was patched to print all test results, even ones
that don't happen, e.g. `(5 passes, 0 failures, 1 errors, 0 skipped,
...)`, but standard unittest prints a terser message that omits test
result types that didn't happen, e.g. `OK (skipped=1)`. Our lit
integration parses this stderr and needs to be updated w/ that
expectation.

I tested this w/ `ninja check-lldb-api` on Linux. There's a good chance
non-Linux tests have similar quirks, but I'm not able to uncover those.

1 files changed, 2 insertions, 2 deletions

diff --git a/lldb/packages/Python/lldbsuite/test/configuration.py b/lldb/packages/Python/lldbsuite/test/configuration.py
index 2a4b9b3..685f491 100644
--- a/lldb/packages/Python/lldbsuite/test/configuration.py
+++ b/lldb/packages/Python/lldbsuite/test/configuration.py
@@ -12,14 +12,14 @@ import os
 
 
 # Third-party modules
-import unittest2
+import unittest
 
 # LLDB Modules
 import lldbsuite
 
 
 # The test suite.
-suite = unittest2.TestSuite()
+suite = unittest.TestSuite()
 
 # The list of categories we said we care about
 categories_list = None