Remove flags from cppflags of libgui_test.

Also fix tests that depend on them.

Rationale: libgui_test currently suffers from ODR violations due to a)
including statically built sources of libgui via the defaults, and b)
loading libgui at runtime via transitive shared library dependency.

The flags set via cppflags affect only a), and this blocks fixing the
ODR violation, which would be done by removing a) and standardizing on
b).  Since flags can be in different stagges of rollout, tests need to
be resilient to encountering either value of the flags.

Flag: EXEMPT refactor
Test: presubmit
Bug: 399711310
Change-Id: I3780eae83facf272f885dc0187c64c13d58e4cdf