Add test requirement to PULL_REQUEST_TEMPLATE.md (#16123)

**What type of PR is this?**
Other

**What does this PR do? Why is it needed?**
This pull request modifies the `PULL_REQUEST_TEMPLATE.md` to ensure the
developer checked that their PR works as expected.

Some contributors push some changes, without even running the modified
client once to see if their changes work as expected.

Avoidable back-and-forth trips between the contributor and the reviewers
could be prevented thanks to running the modified client.

**Acknowledgements**
- [x] I have read
[CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
- [x] I have included a uniquely named [changelog fragment
file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
- [x] I have added a description to this PR with sufficient context for
reviewers to understand this PR.

---------

Co-authored-by: Preston Van Loon <pvanloon@offchainlabs.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -34,4 +34,5 @@ Fixes #
 
 - [ ] I have read [CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
 - [ ] I have included a uniquely named [changelog fragment file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
-- [ ] I have added a description to this PR with sufficient context for reviewers to understand this PR.
+- [ ] I have added a description with sufficient context for reviewers to understand this PR.
+- [ ] I have tested that my changes work as expected and I added a testing plan to the PR description (if applicable). 

changelog/manu-test-pr.md

@@ -0,0 +1,2 @@
+### Ignored
+- Added test requirement to `PULL_REQUEST_TEMPLATE.md`