[9.x] Fix EventFake::assertListening() for asserting string-based observer listeners

[simonhamp]

Follow-up to #42193

Widening the matching mechanism for string-based assertions (from Str::endsWith('@handle') to Str::contains('@')) allowed the underlying logic to unexpectedly handle Observer event assertions, transforming the $actualListener from a string to an array without also transforming the $expectedListener from a string to an array (where the assertion is a string).

(NB: This wasn't breaking any production code, only some tests that used string-based event listener assertion for observers instead of array-based assertion)

This was missed due to a lack of testing around Observer listener assertion in the EventFake test suite. So this PR contains new tests to cover this scenario.

It addresses the issue by explicitly parsing a string-based $expectedListener assertion when the $actualListener is also a string (now always converted to an array as of #42193), so that the $expectedListener correctly matches the parsed $actualListener.

// $actualListener         $expectedListener
Model::observe(new Observer)  'Observer@method'

// Before fix
['Observer', 'method'] === 'Observer@method'
// => false, failing test

// After fix
['Observer', 'method'] === ['Observer', 'method']
// true, pass