Handle GTEST_SKIP() when calling Environment::SetUp()

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>