RFR: JDK-8224011: Failure handling in ExecuteWithLog fails in run-test-prebuilt

[Erik Joelsson]

When running tests using "make run-test-prebuilt", we bypass a lot of 
Init.gmk. One of the things we skip is initialization of the 
make-support/failure-logs dir. This causes make to print errors about 
failing to copy files into that directory if for example the AOT 
compilation fails, partly hiding the original error. This change simply 
adds a MakeDir on that directory as part of the RunTestPrebuilt.gmk 
bootstrap.

Bug: https://bugs.openjdk.java.net/browse/JDK-8224011
Webrev: http://cr.openjdk.java.net/~erikj/8224011/webrev.01/index.html

(The empty change in RunTests.gmk was a trailing tab that my Emacs 
thoroughly disliked, so including that here too.)

/Erik