buck: set vm_args for tests

Maven pom files force the local encoding to UTF-8 to ensure there are
no differences between machines.  They also set the JVM max heap to
256m. Match both in Buck so that results are consistent.

Change-Id: Ice5476dd09352a444a0c97aa0dc28806fddf2ab4

org.eclipse.jgit.pgm.test/BUCK

@@ -22,6 +22,7 @@ for t in TESTS:
       '//lib:tukaani-xz',
     ],
     source_under_test = ['//org.eclipse.jgit.pgm:pgm'],
+    vm_args = ['-Xmx256m', '-Dfile.encoding=UTF-8'],
   )
 
 java_library(

org.eclipse.jgit.test/BUCK

@@ -68,6 +68,7 @@ for src in TESTS:
       '//lib:slf4j-simple',
     ] + DEPS.get(src, []),
     source_under_test = ['//org.eclipse.jgit:jgit'],
+    vm_args = ['-Xmx256m', '-Dfile.encoding=UTF-8'],
   )
 
 java_library(