Template change

[avantgardnerio]

Which issue does this PR close?

Closes #4113.

Rationale for this change

Make it explicit that all PRs should have tests, and force developers to at least ponder why they don't have one.

What changes are included in this PR?

A change to the PR template.

Are there any user-facing changes?

They will be reminded that testing is required.

[alamb]

👍 thank you @avantgardnerio -- love it

[xudong963]

Very good! --- This will reduce the frequency with which maintainers remind contributors to add tests.

[ursabot]

Benchmark runs are scheduled for baseline = b9eee50 and contender = 531dfdc. 531dfdc is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java