Add option to build and run tests during bootstrap

Users that want to enable this option can use the '-t' option to
bootstrap.bash when passing '-r'. Builders that want to enable this can
set the RUN_TESTS environment variable in their bootstrap.bash.

The gotestmain tools is needed to write the main functions for the test
binaries, since 'go test' doesn't work well in this environment.

Change-Id: Iec5c2b5c9c3f5e3ba0ac8677fb88f5e963f9bd3f