Fix creating the test wrapper scripts

The new Makefile rule just dumped more and more lines into the test_x.sh
script, as the ">>" operator was used always.

Makefile.am

@@ -527,6 +527,7 @@ else
 endif
 TESTS = $(check_SCRIPTS)
 %.sh: %
+	@echo "#!/bin/bash" > $@
 	@echo $(wrapper) ./$^ '$$TEST_FLAGS' >> $@
 	@chmod +x $@