[lldb] Modernize TestBitfields

This just does the usual modernizations such as using new test functions where
possible, clang-formatting the source, avoiding manual process setup,
assert improvements (` assertTrue(a == b) -> assertEqual(a, b)`).

This doesn't add any new test cases but removes some dependence on unrelated
features where possible (e.g., structs declared in functions, using the standard
library to printf stuff or initialize objects).