Simplify HasCorrectTimeout

[SimonCropp]

it was never called with a null value. so move it to a calculated property on the attribute

[Evangelink]

Assigning @nohwnd for review

[nohwnd]

The code looks correct, I've initially added it here, as a replacement for repeated calls to GetAttributes,

https://github.com/microsoft/testfx/pull/2839/files

which reduced the numbers of calls to reflection (I think we queried the attributes 4 or 5 times to figure out the timeout).

And then with further optimizations the code was never called with null as pointed out by this PR.

The code where we query the attributes is using the reflection helper methods, so it all should be correct.