RFR(XS): 8050825: Support running regression tests using jtreg_tests+TESTDIRS from top level

[Mikael Vidstedt]

Please review the below change which adds support for running jtreg 
tests from the top level test/ directory using the 'make TESTDIRS=<path> 
jtreg_tests' syntax. The TESTDIRS syntax is already used in files like 
hotspot/test/Makefile and jdk/test/Makefile and allows for selecting 
which jtreg tests to run by providing a directory/path filter. The 
change enables doing the same type of invocation from the top level; 
something like this:

cd test && make TESTDIRS=../hotspot/test/runtime jtreg_tests
cd test && make TESTDIRS=../jdk/test/javax jtreg_tests

The implementation logic simply extracts the component (hotspot, jdk 
etc.) from the value of TESTDIRS and delegates to the respective 
component's test/Makefile, removing the ../<component>/test from 
TESTDIRS in the process.

Thanks,
Mikael