This extends the NotThrow Action assertion by an overload
which allows to specify a wait time. This asserts that the
Action should stop throwing exceptions after the wait time
has passed.

The overload of NotThrow with waiting time is renamed
to NotThrowAfter to emphasize its purpose.

* Change waitTime and pollInterval to TimeSpan
* Use Thread.Sleep instead of Task.Delay
* Add better exception messages if waitTime or pollInterval are out of
  range
* Add tests for invalid argument exceptions
* Simplify tests

* Change local variable names to start with a lower case letter
  in tests of the NotThrowAfter assertion
* Change exception message if NotThrowAfter

The implementation of NotThrowAfter uses Thread.Sleep.
This is not supported in older .net standard versions.
Hence, Thread.Sleep is replaced by a ManualResetEvent
which also allows to wait for a specified time.

Test "NotThrowAfter_when_subject_is_async_it_should_not_throw"
asserts that NotThrowAfter should throw and hence it is renamed
accordingly.

The messages of the exceptions that are raised
if the TimeSpan parameters of NotThrowAfter are
negative claim that the parameters should be
"positive". Changed to "non-negative".

Further change:
* Minor formatting change

This reverts commit b46d678.

NotThrowAfter has to block the current thread.
Since Thread.Sleep() is not available on older versions
of netstandard, the previous commit introduced a workaround.
It seems prefarable not to use this workaround and
disable NotThrowAfter for target frameworks that don't support
Thread.Sleep().

Older versions of netstandard don't support
Thread.Sleep() which is required by NotThrowAfter.

Merge latest changes from upstream

* Add braces to if
* Invert #if condition

Add braces to single line ifs.

For consistency with the ActionAssertions.
Mirrors the implementation for Actions.

This should make sure that exceptions will reference
the correct part of the code.

This should help to avoid test failure on slow systems

Remove async/await which are not necessary here

The wait times of two tests were too low.
This caused the tests to fail on a slow system.

Fixup whitespace

Previously, the return type of NotThrowAfter for Func<T>
was void. Now we return the result of the executed
func to allow for further assertions on that result.