PR template: unit tests checkbox

Author: muzarski

.github/pull_request_template.md

@@ -9,5 +9,6 @@
 - [ ] I have split my patch into logically separate commits.
 - [ ] 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`.