[SPMD] Fix XLA_DUMP_POST_OPTIMIZATIONS test (#5485)
Summary:
XLA_DUMP_POST_OPTIMIZATIONS was set as static which means that the value will be fixed during the whole test run for a particular test suite.
Therefore, let's make a separate file.
Test Plan:
PJRT_DEVICE=TPU USE_XLA_SPMD=1 python test/spmd/test_xla_sharding.py
PJRT_DEVICE=TPU USE_XLA_SPMD=1 python test/spmd/test_xla_sharding_hlo.py
