Adding Kill(bool entireProcessTree) -- terminates a process tree (dotnet/corefx#34147)

Adding Kill(bool entireProcessTree) -- terminates a process tree (dotnet/corefx#34147)

* Adding Kill(bool entireProcessTree)

* Slightly cleaning up NtQueryInformationProcess interop code

* Trimming down PROCESSINFOCLASS enum members

* Refactoring--adjusting indents, changing param from ref to out, etc.

* Revising NtQueryInformationProcess-related code

* Comment & indentation tweaks

* TryGetParentProcessId -> GetParentProcessId rename/refactoring -- includes eliminating the output parameter

* Removing UAP double-definition of KillTree()

* Moving code so that not included in Uap build & tests so that not included in Uap and NetFx builds

* Moving relevant code from Windows -> Win32

* Moving relevant code from Windows -> Win32

* Working to satisfy contract for Uap (code written to fail test to verify that it works)

* Typo fix (missing return signature)

* Removing unneeded Uap-specific test file

* Adding temp. test to verify that UAP CI is catching test failures.

* Using lists (vs. enumerables) as test assert arguments

* Giving Kill() a moment to take effect

* Removing whitespace

* Removing irrelevant comment

* Revising initial check that ensures process has started.

* Moving process tree creation outside of finally

* Adding retry logic to tests

* Adjusting retry times/delay

* Adding backoff to retry helper

* Introducing non-auto-dispose variant of CreateProcess; refactoring

* Improving exception handling

* Moving catch

* Minor refactoring/reformatting

* Testing test execution by CI

* Partially removing code testing CI system

* Experimenting with implementing on OSX

* Adding cast

* Re-adding [ActiveIssue] attribute accidentally removed

* Re-adding inadvertently-removed commented-out test attributes

* Allowing aggregated Win32Exceptions to propagate out; disallowing calling Kill(true) on tree containing caller

* Adjusting test attributes, removing CI verification test, adjusting expected exception

* Marking non-local process test as platform-specific

* Adding PlatformNotSupported throw for Uap.

* Adding missing closing XML tag

* XML doc tag tweak

* Cleaning up two merge artifacts (whitespace + comment)

* Adding console output for debugging

* Fixing typo

* Removing logging to console

* Removing unused parameter

* Unix & OSX exception tweaks

* Variable rename

* Whitespace tweak

* Refactoring IsInTreeOf to use @danmosemsft's optimization suggestion; Renaming method

* Missed a rename

* Tweaking

* Reworking IsSelfOrDescendant to use refactored GetChildProcesses

* Minor fix

* Applying minor tweaks suggested by @krwq

* Adding a delay safety cutoff to RetryWithBackoff per
@krwq's suggestion.

* Adjusting backoff delay max to 10 seconds.

* IsSelfOrDescendant -> IsSelfOrDescendantOf

* Unix: If Stop() fails, don't try to continue with the process (thanks @tmds!).

* Fixing typo.

* Adding comments about kill/stop returning before termination/stopping completes.

* Adding comment.

Commit migrated from https://github.com/dotnet/corefx/commit/81bd671efda5db20fae1eb381aeff2cf8ea727ac