[netcore] Review and document netcore test failures that are skipped (mono/mono#15359)

[netcore] Review and document netcore test failures that are skipped (mono/mono#15359)

* Went through the majority of corefx tests that were being skipped in CoreFX.issues.rsp.

For each failure, a github issue was logged and if possible, a more expanded
failure reason was specified.

Part of the https://github.com/mono/mono/issues/14787 epic work

* Cleaned up header that no longer had tests in it

* Pesky -method still around.  It'll ignore now

Commit migrated from https://github.com/mono/mono/commit/814d20db9155ec568009e7f4374f386bcfd32977