Skip core tests when not core testing

[ethanwharris]

What does this PR do?

Currently, all CI workflows run all of the tests that do not have specific requirements ('core' tests).
WIth this PR, the core tests will only run in core workflows, speeding up the CI and reducing issues created by test flakiness.
This PR additionally filters some serve tests into their own workflow too.

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

• Is this pull request ready for review? (if not, please submit in draft mode)

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #1330 (3a3797e) into master (3faf9f6) will increase coverage by 9.82%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1330      +/-   ##
==========================================
+ Coverage   81.65%   91.48%   +9.82%     
==========================================
  Files         287      287              
  Lines       12846    12881      +35     
==========================================
+ Hits        10490    11784    +1294     
+ Misses       2356     1097    -1259     

Flag	Coverage Δ
unittests	91.48% <100.00%> (+9.82%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/core/serve/utils.py	100.00% <ø> (+3.44%)	⬆️
flash/core/data/data_module.py	94.23% <100.00%> (+0.48%)	⬆️
flash/core/data/utilities/classification.py	98.27% <100.00%> (+1.19%)	⬆️
flash/core/data/utils.py	89.47% <100.00%> (+0.28%)	⬆️
flash/core/model.py	88.91% <100.00%> (+0.05%)	⬆️
flash/core/optimizers/lamb.py	84.93% <100.00%> (+0.64%)	⬆️
flash/core/optimizers/lars.py	82.75% <100.00%> (+0.94%)	⬆️
flash/core/optimizers/lr_scheduler.py	93.75% <100.00%> (+0.64%)	⬆️
flash/core/serve/dag/optimization.py	93.77% <100.00%> (-0.59%)	⬇️
flash/core/serve/dag/order.py	96.40% <100.00%> (-0.37%)	⬇️
... and 86 more

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[krshrimali]

Nice! 🚀 LGTM 😃