Ian suggested that since test caching is not expected to be perfect
in all cases, we should allow users to clear the test cache separately
from clearing the entire build cache.

This CL adds 'go clean -testcache' to do that. The implementation
does not actually delete files (for that, use 'go clean -cache').
Instead, it writes down the current time, and future go tests will
ignore any cached test results written before that time.

Change-Id: I4f84065d7dfc2499fa3f203e9ab62e68d7f367c5
Reviewed-on: https://go-review.googlesource.com/78176Reviewed-by: Ian Lance Taylor <iant@golang.org>