pub run build_runner test runs tests even if build has SEVERE errors

[chalin]

I'm not sure if this is related to #910, but pub run build_runner test runs tests even if build has SEVERE errors. I don't think any attempt should be made to run tests if there are build errors.

This should even hold true when repeated attempts are made to run tests. I.e., if you start with a clean project repo, then execute pub run build_runner test and there are build errors then tests should not run.

But also, if I rerun pub run build_runner test (and no source files have changed), the test runner should know that the build failed and refuse to run tests (IMHO).

cc @kwalrath

[jakemac53]

Merging with #910