[Windows Sandbox Tests] Re-format process mitigation tests.

- Rename process_mitigations_test.cc -> process_mitigations_unittest.cc
- Pull large chunks out of process_mitigations_unittest.cc into unittest
files of their own.  The size was getting way out of hand.
- New files for existing tests: extensionpoints, imageload, win32k.
- NO new functionality tests in this CL - only re-organized things:
  - Note: the existing tests for every process mitigation that just check policy
	enabling have been centralized into one single test target function
	(SBOX_TESTS_COMMAND) called CheckPolicy.  Any new mitigation checks
	should be added here as well.
  - Note: The individual mitigation TESTs that trigger "CheckPolicy" have been
	adjusted to ensure we test both pre and post child-process startup wherever
	they are supported.

TEST= sbox_integration_tests.exe, ProcessMitigationsTest.*

Review-Url: https://codereview.chromium.org/2944493002
Cr-Commit-Position: refs/heads/master@{#485387}
12 files changed