Fix #892 by improving test code

[vgnshiyer]

(Describe the goal of this PR. Mention any related Issue numbers)

Fixes #892

This PR handles the exception when asyncio fails to get event loop from the main thread when the codecov CI job is run. Current changes tries to get an event loop, else it creates a new one and returns it.

Category (place an x in each of the [ ])

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements (place an x in each [ ])

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

Merging #918 (e7953e8) into main (f8c1b86) will increase coverage by 0.03%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##             main     #918      +/-   ##
==========================================
+ Coverage   91.74%   91.77%   +0.03%     
==========================================
  Files         181      181              
  Lines        6285     6309      +24     
==========================================
+ Hits         5766     5790      +24     
  Misses        519      519              

Impacted Files	Coverage Δ
slack_bolt/app/async_app.py	94.16% <90.00%> (+0.08%)	⬆️
slack_bolt/app/app.py	87.44% <100.00%> (+0.16%)	⬆️
slack_bolt/authorization/authorize_result.py	95.12% <100.00%> (+1.18%)	⬆️
slack_bolt/context/respond/async_respond.py	92.59% <100.00%> (ø)
slack_bolt/context/respond/internals.py	100.00% <100.00%> (ø)
slack_bolt/context/respond/respond.py	92.59% <100.00%> (ø)
slack_bolt/error/__init__.py	100.00% <100.00%> (ø)
slack_bolt/version.py	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[seratch]

Perfect! Thank you 🙇