build(gulp): fix concurrency and caching issues in test.unit.js and test.unit.dart tasks

previously there was a chance of race conditions that could sporadically fail the build.

additionally runing a task via gulp.start or runSequence always reruns its dependencies, which meant that we were blowing away
the build.tools build and rebuilding everything from scratch even during the interactive/watch mode. This meant that the build
pipeline cache was destroyed on every change and we never got the benefit of incremental compilation