use GRAALPYTHON_GRAALVM_SUPPORT as home for running junit tests

Author: timfel

doc/CONTRIBUTING.md

@@ -129,7 +129,7 @@ To run the JUnit tests, you can use this command:
 To run a subset of the tests, you can use the following. Again, you can use `-d`
 to attach with a Java debugger.
 
-    mx [-d] unittest JAVA-TEST-CLASSNAME
+    mx [-d] punittest JAVA-TEST-CLASSNAME
 
 To run the Python standard library tests, you can use the following:
 

mx.graalpython/mx_graalpython.py

@@ -170,7 +170,8 @@ def punittest(ars):
              "-Dgraal.TruffleCompilationExceptionsArePrinted=true",
              "-Dgraal.TrufflePerformanceWarningsAreFatal=false"]
     args += ars
-    mx_unittest.unittest(args)
+    with set_env(GRAAL_PYTHONHOME=mx.dependency("GRAALPYTHON_GRAALVM_SUPPORT").get_output()):
+        mx_unittest.unittest(args)
 
 
 PYTHON_ARCHIVES = ["GRAALPYTHON_GRAALVM_SUPPORT"]