do not hard-code number of jobs to 1 when running ctest

This allows running tests in parallel using the --ctest-args option with an
invocation like this:

build.py ... --stages test --ctest-args "--parallel 8" -- .

build.py

@@ -287,19 +287,15 @@ def RunCMake(context, extraArgs=None, stages=None):
 def RunCTest(context, extraArgs=None):
     buildDir = context.buildDir
     variant = BuildVariant(context)
-    #TODO we can't currently run tests in parallel, something to revisit.
-    numJobs = 1
 
     with CurrentWorkingDirectory(buildDir):
         Run(context,
             'ctest '
             '--output-on-failure ' 
             '--timeout 300 '
-            '-j {numJobs} '
             '-C {variant} '
             '{extraArgs} '
-            .format(numJobs=numJobs,
-                    variant=variant,
+            .format(variant=variant,
                     extraArgs=(" ".join(extraArgs) if extraArgs else "")))
 
 def RunMakeZipArchive(context):