Remove nonexistent tests dir from BlinkHeapUnitTests.isolate.

TBR=navabi@chromium.org
BUG=334018

Review URL: https://codereview.chromium.org/148773005

git-svn-id: svn://svn.chromium.org/blink/trunk@166328 bbb929c8-8fbe-4397-9dbb-9b2b20218538