[lldb][test] Re-enable `TestRerunAndExprDylib.py` (#157872)

the `skipTestIfFn` requires a function that return a string to skip or
None to run the test. The `isUbuntu18_04` function returns a bool and
the test is skipped on all platforms.

https://github.com/llvm/llvm-project/blob/25ebdfe0ab202a6cb30232d84bc5838439fd67d5/lldb/packages/Python/lldbsuite/test/decorators.py#L145-L157

GitOrigin-RevId: f5315bd135b6e65ebfb0d27f6333501c3a4b32b3