Makefile: target 'tests' picks up extra arguments via TESTS_ARGS

TESTS_ARGS options are passed to prism-auto.

E.g.:
make tests TESTS_ARGS="-x '-epsilon 1e-10'"

prism/Makefile

@@ -413,21 +413,24 @@ test:
 	bin/prism ../prism-tests/functionality/verify/dtmcs/dtmc_pctl.pm ../prism-tests/functionality/verify/dtmcs/dtmc_pctl.pm.props -prop 2 -test
 
 # Run all tests (in ../prism-tests and ./tests)
+# Optionally, extra arguments for prism-auto are picked up via variable TESTS_ARGS
 tests: testslocal
 	@if [ -d ../prism-tests ]; then \
-	  cd ../prism-tests && "$(PWD)"/etc/scripts/prism-auto -t -m . -p "$(PWD)"/bin/prism --nailgun --ngprism "$(PWD)"/bin/ngprism; \
+	  cd ../prism-tests && "$(PWD)"/etc/scripts/prism-auto -t -m . -p "$(PWD)"/bin/prism --nailgun --ngprism "$(PWD)"/bin/ngprism $(TESTS_ARGS); \
 	else \
 	  echo "Skipping tests"; \
 	fi
 
 # Just display the command to run the test suite on this version of PRISM
+# Optionally, extra arguments for prism-auto are picked up via variable TESTS_ARGS
 testsecho:
-	@echo etc/scripts/prism-auto -t -m --nailgun --ngprism bin/ngprism ../prism-tests -p bin/prism
+	@echo etc/scripts/prism-auto -t -m --nailgun --ngprism bin/ngprism ../prism-tests -p bin/prism $(TESTS_ARGS)
 
 # Run local tests (in ./tests)
+# Optionally, extra arguments for prism-auto are picked up via variable TESTS_ARGS
 testslocal:
 	@if [ -d tests ]; then \
-	  cd tests && "$(PWD)"/etc/scripts/prism-auto -t -m . -p "$(PWD)"/bin/prism --nailgun --ngprism "$(PWD)"/bin/ngprism; \
+	  cd tests && "$(PWD)"/etc/scripts/prism-auto -t -m . -p "$(PWD)"/bin/prism --nailgun --ngprism "$(PWD)"/bin/ngprism $(TESTS_ARGS); \
 	else \
 	  echo "Skipping local tests"; \
 	fi