Remove the use of ExpectedException in Mono.Posix Tests (#4698)

Remove the use of ExpectedException in Mono.Posix Tests (#4698)

* Remove the use of ExpectedException in Mono.Posix Tests

ExpectedException is not available in later versions of
NUnit.  To test the NetCore 2.0 version of Mono.Posix I
need a newer version of NUnit that does not contain ExpectedException.

Using Assert.Throws should be equivalent.

* Mono.Posix Tests move CanUseRealTimeSignals checks

Moving the CanUseRealTimeSignals check outside of the
Assert.Throws callback function.

This is a change in test behavior requested by @akoeplinger
in the PR review.

* Fixing Mono.Posix.UnixSignalTest.TestSignumPropertyThrows

This test throws InvalidOperationException and we need to check
for that in the Assert.Throws type.