Replace 'Matchers' with 'CoreMatchers' So we do not require 'org.hamcrest:hamcrest-library' as additional dependency anymore. Which was only available for most of the tests because it leaked onto the test compile classpath.
Use external repository mirrors in build (#6094) We've been bitten by external repository fluctuation for a long time. This PR makes most of tests use repository mirrors set up by ourselves via init script and system property.
There're still some tests not switching to mirrors, which would be fixed in follow-up commits.