pytest: Support targeting specific tests using make pytest.

Select tests by setting the `PYTEST_TESTS` environment variable.

Changelog-None

Makefile

@@ -456,6 +456,11 @@ else
 PYTEST_OPTS += -x
 endif
 
+# Allow for targeting specific tests by setting the PYTEST_TESTS environment variable.
+ifeq ($(PYTEST_TESTS),)
+PYTEST_TESTS = "tests/"
+endif
+
 check-units:
 
 check: check-units installcheck pytest
@@ -466,7 +471,7 @@ ifeq ($(PYTEST),)
 	exit 1
 else
 # Explicitly hand VALGRIND so you can override on make cmd line.
-	PYTHONPATH=$(MY_CHECK_PYTHONPATH) TEST_DEBUG=1 VALGRIND=$(VALGRIND) $(PYTEST) tests/ $(PYTEST_OPTS)
+	PYTHONPATH=$(MY_CHECK_PYTHONPATH) TEST_DEBUG=1 VALGRIND=$(VALGRIND) $(PYTEST) $(PYTEST_TESTS) $(PYTEST_OPTS)
 endif
 
 check-fuzz: $(ALL_FUZZ_TARGETS)