Add test-one Makefile target for running individual tests (#109)

`make test-one t=TestName` runs a single test by name using
ponytest's --only flag. Useful for faster iteration when
debugging or developing a specific test.

Author: SeanTAllen

Makefile

@@ -51,6 +51,9 @@ test: unit-tests examples
 unit-tests: $(tests_binary)
 	$^ --exclude=integration --sequential
 
+test-one: $(tests_binary)
+	$^ --only="$(t)"
+
 $(tests_binary): $(SOURCE_FILES) | $(BUILD_DIR) fetch
 	$(PONYC) -o $(BUILD_DIR) $(SRC_DIR)
 
@@ -84,4 +87,4 @@ all: test
 $(BUILD_DIR):
 	mkdir -p $(BUILD_DIR)
 
-.PHONY: all examples _build_examples clean fetch TAGS test
+.PHONY: all examples _build_examples clean fetch TAGS test test-one