RFR(XS) 8212221: GraalUnitTestLauncher should generate MX_SUBPROCESS_COMMAND_FILE

[Ekaterina Pavlova]

Hi,

Some graal unit tests rely on MX_SUBPROCESS_COMMAND_FILE env variable which contains
name of the file with java path and java args used to launch current test process.
This change creates such file and sets MX_SUBPROCESS_COMMAND_FILE variable.
Please review.

     JBS: https://bugs.openjdk.java.net/browse/JDK-8212221
  webrev: http://cr.openjdk.java.net/~epavlova//8212221/webrev.00/index.html
testing: previously failed tests were removed from ProblemList-graal.txt and executed as part of graal unit testing in Mach5.

thanks,
-katya