allow to run only a subset of the runtime tests

with
	make TESTS='test_1 test_2 ...' regress

now it's possible to run only that specified subset of tests.  It's
really useful during debugging :)

Makefile

@@ -1,3 +1,7 @@
+# tests to run as a port of the regression suite.  Leave empty to run
+# all.
+TESTS=
+
 .PHONY: all static clean regress install
 
 all: Makefile.local gmid TAGS compile_flags.txt
@@ -29,7 +33,7 @@ clean:
 	${MAKE} -C regress clean
 
 regress: gmid
-	${MAKE} -C regress all
+	${MAKE} TESTS=${TESTS} -C regress all
 
 install: gmid
 	mkdir -p ${DESTDIR}${BINDIR}

regress/Makefile

@@ -1,3 +1,7 @@
+# tests to run as a port of the regression suite.  Leave empty to run
+# all.
+TESTS=
+
 include ../Makefile.local
 
 .PHONY: all data clean runtime
@@ -5,7 +9,7 @@ include ../Makefile.local
 all: data gg puny-test fcgi-test
 	./puny-test
 	./iri_test
-	./runtime
+	./runtime ${TESTS}
 
 data: testdata iri_test cert.pem testca.pem valid.crt invalid.cert.pem