Disable Soong tests by default on Linux

Disable running the Soong tests when building on Linux.  The tests
are still enabled when running on Darwin to ensure we have test on
Darwin in CI.  Checkbuilds still depend on the tests, which will
provide test coverage in CI on Linux.

Bug: 269296618
Test: aninja -t path checkbuild out/host/linux-x86/bin/go/soong-java/test/test.passed
Test: m nothing
Change-Id: Ib8359ff6d2a92eeb2ac9a9e95ecd65abc4e40061