4.x: Refactor Linq operator unit tests into separate files/classes

[akarnokd]

This PR refactors the various operator unit tests into their separate files and allows:

• locating tests of an operator by name instead of a category,
• running tests of one operator instead of a group of operators.

[akarnokd]

Now the fun part. Finding out which are those 2 tests that got lost:

[clairernovotny]

@akarnokd that could be due to the removal of some of the TFM's. We removed netcoreapp1.1, so that could account for one test depending on what the base version is.

[akarnokd]

I was verifying periodically if the test runner still has the original 3534 test listed and it still shows me that:

that could be due to the removal of some of the TFM's.

This shows 7010 as well:

[danielcweber]

Can you build it locally through the script, it should open a coverage report at the end, if it didn't decrease, it'll be good to merge.

[danielcweber]

Ok if the mystery of the lost tests is solved.

[akarnokd]

Looks like there were changes on the master earlier and I compared against another PR probably pointing further back in time.