make running specific unit tests easier (#4336)

I find it's easier to do TDD if i can run specific unit tests. I know
watch is there but some people prefer running their tests manually.

Makefile

@@ -32,11 +32,13 @@ lint lint_diff:
 	poetry run black $(PYTHON_FILES) --check
 	poetry run ruff .
 
+TEST_FILE ?= tests/unit_tests/
+
 test:
-	poetry run pytest tests/unit_tests
+	poetry run pytest $(TEST_FILE)
 
 tests:
-	poetry run pytest tests/unit_tests
+	poetry run pytest $(TEST_FILE)
 
 test_watch:
 	poetry run ptw --now . -- tests/unit_tests
@@ -50,13 +52,14 @@ docker_tests:
 
 help:
 	@echo '----'
-	@echo 'coverage            - run unit tests and generate coverage report'
-	@echo 'docs_build          - build the documentation'
-	@echo 'docs_clean          - clean the documentation build artifacts'
-	@echo 'docs_linkcheck      - run linkchecker on the documentation'
-	@echo 'format              - run code formatters'
-	@echo 'lint                - run linters'
-	@echo 'test                - run unit tests'
-	@echo 'test_watch          - run unit tests in watch mode'
-	@echo 'integration_tests   - run integration tests'
-	@echo 'docker_tests        - run unit tests in docker'
+	@echo 'coverage                     - run unit tests and generate coverage report'
+	@echo 'docs_build                   - build the documentation'
+	@echo 'docs_clean                   - clean the documentation build artifacts'
+	@echo 'docs_linkcheck               - run linkchecker on the documentation'
+	@echo 'format                       - run code formatters'
+	@echo 'lint                         - run linters'
+	@echo 'test                         - run unit tests'
+	@echo 'test TEST_FILE=<test_file>   - run all tests in file'
+	@echo 'test_watch                   - run unit tests in watch mode'
+	@echo 'integration_tests            - run integration tests'
+	@echo 'docker_tests                 - run unit tests in docker'