Fix xUnit & MsTest message when exception will be thrown inside lambda.

[CaptnCodr]

This fixes the output message described in #214.

MsTest message was:

Message: 
    FsUnit.Test.TestException "help" was <fun:Pipe #1 input at line 80@80>
Stack Trace: 
    raise tests.Test raising exception with message() line 81

is now e.g.:

Message: 
    FsUnit.Test.TestException "help" was "System.Exception: help
        at <StartupCode$FsUnit-MsTest-Test>.$RaiseTests.Pipe #1 input at line 80@80.Invoke(Unit unitVar0) in C:\Users\const\github\repos\FsUnit\tests\FsUnit.MsTest.Test\raiseTests.fs:line 80
        at FsUnit.Test.raise tests.Test raising exception with message() in C:\Users\const\github\repos\FsUnit\tests\FsUnit.MsTest.Test\raiseTests.fs:line 81"

Stack Trace: 
    raise tests.Test raising exception with message() line 81

---

xUnit message was:

Message: 
    FsUnit.Xunit+MatchException : Exception of type 'FsUnit.Xunit+MatchException' was thrown.
    Expected: FsUnit.Test.TestException "help"
    Actual:   <fun:Pipe #1 input at line 79@79>

Stack Trace: 
    That.Static[a](a actual, IMatcher`1 matcher) line 28
    raise tests.Test raising exception with message() line 80

is now e.g.:

Message: 
    FsUnit.Xunit+MatchException : Exception of type 'FsUnit.Xunit+MatchException' was thrown.
    Expected: FsUnit.Test.TestException "help"
    Actual:   System.Exception: help
                at <StartupCode$FsUnit-Xunit-Test>.$RaiseTests.Pipe #1 input at line 79@79.Invoke(Unit unitVar0) in C:\Users\const\github\repos\FsUnit\tests\FsUnit.Xunit.Test\raiseTests.fs:line 79
                at FsUnit.Xunit.Assert.That.Static[a](a actual, IMatcher`1 matcher) in C:\Users\const\github\repos\FsUnit\src\FsUnit.Xunit\FsUnit.fs:line 22

Stack Trace: 
    That.Static[a](a actual, IMatcher`1 matcher) line 26
    raise tests.Test raising exception with message() line 80

---

Beyond that, I refactored operator equalWithin a bit because of that comment above that function

Edit: the output of NUnit is not affected by this change, this is already covered by the NUnit.Framework.

[CaptnCodr]

@sergey-tihon Do you have any corrections/suggestions here? 🙂

[sergey-tihon]

no, looks good to me.
Thank you for fixing this.