[TF:XLA] Don't remove TF_XLA_FLAGS when enabling autojit in _xla_gpu tests.
[TF:XLA] Don't remove TF_XLA_FLAGS when enabling autojit in _xla_gpu tests.

Previously, the environment variable was overwritten with the test specific flags. Now they are concatenated together.

PiperOrigin-RevId: 236275779

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