PR template: mention Makefile for test filters

The tests are no longer configured in the GitHub workflows, but in
the Makefile. This change updates the PR template to reflect that.

Author: wprzytula

.github/pull_request_template.md

@@ -10,5 +10,4 @@
 - [ ] All commit messages clearly explain what they change and why.
 - [ ] PR description sums up the changes and reasons why they should be introduced.
 - [ ] I have implemented Rust unit tests for the features/changes introduced.
-- [ ] I have enabled appropriate tests in `.github/workflows/build.yml` in `gtest_filter`.
-- [ ] I have enabled appropriate tests in `.github/workflows/cassandra.yml` in `gtest_filter`.
+- [ ] I have enabled appropriate tests in `Makefile` in `{SCYLLA,CASSANDRA}_(NO_VALGRIND_)TEST_FILTER`.