Allow running specific tests by passing TESTS

Make it possible to specify individual tests to run, to facilitate
debugging. Running the entire test suite every time can take quite long.
Tests can be specified by passing the TESTS variable to `make test`.

If TESTS is non-empty, the test script defines the `suite` hook to build
the test suite using `suite_addTest` with each specified test function.
If TESTS is unset or empty, shUnit2's default behaviour is to run all
functions beginning with the word `test`.

See https://github.com/kward/shunit2#-suites

Author: cjolowicz

Makefile

@@ -10,17 +10,17 @@ check:
 
 test-cedar-14:
 	@echo "Running tests in docker (cedar-14)..."
-	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=cedar-14" heroku/cedar:14 bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run-deps; test/run-features; test/run-versions;'
+	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=cedar-14" -e TESTS=$(TESTS) heroku/cedar:14 bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run-deps; test/run-features; test/run-versions;'
 	@echo ""
 
 test-heroku-16:
 	@echo "Running tests in docker (heroku-16)..."
-	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=heroku-16" heroku/heroku:16-build bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run-deps; test/run-features; test/run-versions;'
+	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=heroku-16" -e TESTS=$(TESTS) heroku/heroku:16-build bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run-deps; test/run-features; test/run-versions;'
 	@echo ""
 
 test-heroku-18:
 	@echo "Running tests in docker (heroku-18)..."
-	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=heroku-18" heroku/heroku:18-build bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run-deps; test/run-features; test/run-versions;'
+	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=heroku-18" -e TESTS=$(TESTS) heroku/heroku:18-build bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run-deps; test/run-features; test/run-versions;'
 	@echo ""
 
 buildenv-heroku-16:

README.md

@@ -83,6 +83,12 @@ make test-heroku-18
 make test-heroku-16
 ```
 
+You can also specify which tests to run:
+
+```
+make test TESTS="testPython3_7 testGitEgg"
+```
+
 The tests are run via the vendored
 [shunit2](https://github.com/kward/shunit2)
 test framework.

test/utils

@@ -264,3 +264,13 @@ release() {
 assertFile() {
   assertEquals "$1" "$(cat ${compile_dir}/$2)"
 }
+
+# If TESTS is present in the environment, only run the specified tests.
+if [ -n "$TESTS" ]; then
+    suite() {
+        for shunit_func_ in ${TESTS}; do
+            suite_addTest ${shunit_func_}
+        done
+        unset shunit_func_
+    }
+fi