Add make run-tests RUN=TEST variable.

It is often useful to run only a single test instead of running all
the tests. This can be done with `nunit-console -run:TESTNAME`.

Allow the run-tests target to use the RUN variable to control the
`nunit-console -run` option:

	make run-tests RUN=Java.InteropTests.InvokeVirtualFromConstructorTests

Author: jonpryor

Makefile

@@ -51,6 +51,7 @@ define RUN_TEST
 	_JI_LOG=gref=g-$(basename $(notdir $(1))).txt,$(if $(2),lref=l-$(basename $(notdir $(1))).txt,) \
 	mono --debug=casts $$MONO_OPTIONS --runtime=v4.0.0 \
 		lib/NUnit-2.6.3/bin/nunit-console.exe $(1) \
+		$(if $(RUN),-run:$(RUN)) \
 		-output=bin/$(CONFIGURATION)/TestOutput-$(basename $(notdir $(1))).txt ;
 endef