add flag to skip soong tests for b command

Sometimes it is very handy during testing to temporarily skip Soong
tests, but there is not easy method to allow this without changing the
envsetup.sh file and re-sourcing before/after enabling/disabling the
skip-tests flag.

Test: b build '...' --skip-soong-tests
Change-Id: I6dcad3ba0336cfd1c4d2c4357b852900792445c3