Restore Makefile define + call pattern for cleaning tests

[nicksay]

When running all tests in sequence, removing previously generated files is needed to ensure proper output. A previous cleanup of the Makefile changed targets from:

test_target_one:
  $(call _clean_tests)
  ...
test_target_two:
  $(call _clean_tests)
  ...

to:

test_target_one: clean_tests
test_target_two: clean_tests
  ...

However, this change meant that the new Make dependency order was:

• clean_tests
• test_target_one
• test_target_two

instead of:

• clean_tests
• test_target_one
• clean_tests
• test_target_two

To fix this, restore the previous define + call pattern.

Progress on #37

[awbraunstein]

lgtm