(v2) tea.go: deadlock/error return fixes, additional tests (targeted at v2)

[desertwitch]

This PR contains the v2-targeted and v2-applicable portions of the following v1-targeted fixes:

• (v1-0/3) tea.go: all fixes combined (for convenience/CI) #1376
• (v1-1/3) tea.go: fix deadlock on context cancellation #1372
• (v1-2/3) tea.go: teardown-related deadlock & race condition fixes #1373
• (v1-3/3) tea.go: fix panic handling race condition + wrongful nil returns despite errors (ctx, panic, ...) #1375

Meaning it fixes all above issues in v2, the bugs that are not already resolved otherwise in v2.
Issues are related to deadlocks and wrongful nil returns on error, PR adds a couple of tests as well.

[codecov]

Codecov Report

Attention: Patch coverage is 98.21429% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (v2-exp@7858a14). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
tea.go	98.18%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             v2-exp    #1388   +/-   ##
=========================================
  Coverage          ?   47.95%           
=========================================
  Files             ?       26           
  Lines             ?     1810           
  Branches          ?        0           
=========================================
  Hits              ?      868           
  Misses            ?      877           
  Partials          ?       65           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.