test: add a test for canceling the context

[caarlos0]

this adds test for canceling the context within the bubble

[desertwitch]

This PR (and included tests) should be obsolete and resolved with the proposed changes in #1372.

The behavior documented in this PR:

// Note that canceling the context from within bubbletea (e.g. in an [Cmd]) is
// not supported.

Actually is a symptom of a deadlock issue described in #1370 and resolved through #1372 (when merged).
By resolving the deadlock issue, cancellation from within bubbletea is possible (as it is from outside and should be).