Import proof-of-concept for handling `GTEST_SKIP()` in `Environment::SetUp`

Import proof-of-concept for handling `GTEST_SKIP()` in `Environment::SetUp`

Per the upstream pull-request [1]:

```
  gtest prior to this change would completely ignore `GTEST_SKIP()` if
  called in `Environment::SetUp()`, instead of bailing out early, unlike
  `Test::SetUp()`, which would cause the tests themselves to be skipped.
  The only way (prior to this change) to skip the tests would be to
  trigger a fatal error via `GTEST_FAIL()`.

  Desirable behavior, in this case, when dealing with
  `Environment::SetUp()` is to check for prerequisites on a system
  (example, kernel supports a particular featureset, e.g., capsicum), and
  skip the tests. The alternatives prior to this change would be
  undesirable:

  - Failing sends the wrong message to the test user, as the result of the
    tests is indeterminate, not failed.
  - Having to add per-test class abstractions that override `SetUp()` to
    test for the capsicum feature set, then skip all of the tests in their
    respective SetUp fixtures, would be a lot of human and computational
    work; checking for the feature would need to be done for all of the
    tests, instead of once for all of the tests.

  For those reasons, making `Environment::SetUp()` handle `GTEST_SKIP()`,
  by not executing the testcases, is the most desirable solution.

  In order to properly diagnose what happened when running the tests if
  they are skipped, print out the diagnostics in an ad hoc manner.

  Update the documentation to note this change and integrate a new test,
  gtest_skip_in_environment_setup_test, into the test suite.

  This change addresses #2189.

  Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>
```

The goal with my merging in this change is to avoid requiring extensive
refactoring/retesting of test suites when ensuring prerequisites are met,
e.g., checking for a CAPABILITIES-enabled kernel before running capsicum-test
(see D19758 for more details).

The proof-of-concept is being imported before accepted by the upstream
project due to the fact that the upstream project is undergoing a potential
development freeze and the maintainers aren't responding to my PR.

1. https://github.com/google/googletest/pull/2203

Reported by:	asomers (https://github.com/google/googletest/issues/2189)
Reviewed by:	asomers
Approved by:	emaste (mentor)
MFC after:	2 months
Differential Revision:	https://reviews.freebsd.org/D19765