Edit PR template to bolster test requirements & tighten prose

[rouson]

Avg response time	coverage on master

Summary of changes

Edited the PR template.

Rationale for changes

The modified test requirements are a bit more stringent to help us ensure monotonically increasing test coverage as features get added and bugs get fixed.

Additional info and certifications

This pull request (PR) is a:

• Bug fix
• Feature addition
• Other, Please describe: contribution requirements update.

I certify that

• I have reviewed the contributing guidelines and followed the
  policies on:
  - Pull request (PR) naming to indicate work in progress (WIP),
  and to attach the PR to the appropriate bug report, or feature
  request issue
  - White space (no trailing white space or white space errors may
  be introduced)
  - Commenting code where it is non-obvious and non-trivial
  - Running tests locally, to ensure all of them pass
  - Maintaining or increasing test coverage
  - Logically atomic, self consistent and coherent commits
  - Commit message content
  - Waiting 24 hours before self-approving the pull request to
  give another OpenCoarrays developer a chance to review my
  proposed code

[zbeekman]

@rouson: We could include codecov.io requirements for monotonically increasing coverage too, to programmatically enforce parts of this. In practice it might be a nuisance, but if you want to try this please let me know and I can adjust the yaml file and GH repository settings accordingly.

LGTM

[codecov]

Codecov Report

Merging #431 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #431   +/-   ##
=======================================
  Coverage   39.38%   39.38%           
=======================================
  Files           3        3           
  Lines        2105     2105           
  Branches      350      350           
=======================================
  Hits          829      829           
  Misses       1154     1154           
  Partials      122      122