[gwp_asan] Soft-transition ZXTEST_USE_STREAMABLE_MACROS removal (#121887)

Soft-transition the removal of setting ZXTEST_USE_STREAMABLE_MACROS, by
only setting the macro if not already defined. A future PR will remove
setting the macro entirely in harness.h.

GitOrigin-RevId: 478648e2c0adbafa5e4cb10eb53aadbdf7de97d9
1 file changed