Fail tests on exceptions raised after test completed (#144706)

A test was failing silently because of this (see
https://github.com/flutter/flutter/issues/144353 and fixed in
https://github.com/flutter/flutter/pull/144709). The failure went
undetected for months. Ideally, this should have been a regular
non-silent failure. This change makes that so. `package:test` can
properly handle reported exceptions outside of test cases. With this
change, the test fails as follows:

```
00:03 +82: Smoke test material/color_scheme/dynamic_content_color.0.dart
══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following assertion was thrown running a test (but after the test had completed):
setState() called after dispose(): _DynamicColorExampleState#1cd37(lifecycle state: defunct, not
mounted)
This error happens if you call setState() on a State object for a widget that no longer appears in
the widget tree (e.g., whose parent widget no longer includes the widget in its build). This error
can occur when code calls setState() from a timer or an animation callback.
The preferred solution is to cancel the timer or stop listening to the animation in the dispose()
callback. Another solution is to check the "mounted" property of this object before calling
setState() to ensure the object is still in the tree.
This error might indicate a memory leak if setState() is being called because another object is
retaining a reference to this State object after it has been removed from the tree. To avoid memory
leaks, consider breaking the reference to this object during dispose().

When the exception was thrown, this was the stack:
#0      State.setState.<anonymous closure> (package:flutter/src/widgets/framework.dart:1167:9)
#1      State.setState (package:flutter/src/widgets/framework.dart:1202:6)
#2      _DynamicColorExampleState._updateImage (package:flutter_api_samples/material/color_scheme/dynamic_content_color.0.dart:191:5)
<asynchronous suspension>
════════════════════════════════════════════════════════════════════════════════════════════════════
00:03 +81 -1: Smoke test material/context_menu/context_menu_controller.0.dart
00:03 +81 -1: Smoke test material/color_scheme/dynamic_content_color.0.dart [E]
  Test failed. See exception logs above.
  The test description was: Smoke test material/color_scheme/dynamic_content_color.0.dart

  This test failed after it had already completed.
  Make sure to use a matching library which informs the test runner
  of pending async work.
```
https://dart.googlesource.com/external/github.com/flutter/flutter/+/91cccc8e6b83ada2dcffe266ff4a6d3c8d0ea582