test/librbd: use GTEST_SKIP macro to skip tests

test/librbd: use GTEST_SKIP macro to skip tests

The use of SUCCEED macro predates the introduction of GTEST_SKIP macro
to GTest 1.10.  Having skipped tests reported as passed is misleading!

Before:

[ RUN      ] TestMockOperationSnapshotRemoveRequest.FlattenedCloneRemovesChild
SKIPPING
[       OK ] TestMockOperationSnapshotRemoveRequest.FlattenedCloneRemovesChild (9 ms)
...
[ RUN      ] TestMockOperationSnapshotRemoveRequest.RemoveChildError
SKIPPING
[       OK ] TestMockOperationSnapshotRemoveRequest.RemoveChildError (112 ms)
...
[  PASSED  ] 16 tests.

After:

[ RUN      ] TestMockOperationSnapshotRemoveRequest.FlattenedCloneRemovesChild
../src/test/librbd/operation/test_mock_SnapshotRemoveRequest.cc:381: Skipped
Skipping due to unmet REQUIRE
[  SKIPPED ] TestMockOperationSnapshotRemoveRequest.FlattenedCloneRemovesChild (9 ms)
...
[ RUN      ] TestMockOperationSnapshotRemoveRequest.RemoveChildError
../src/test/librbd/operation/test_mock_SnapshotRemoveRequest.cc:727: Skipped
Skipping due to enabled deep-flatten
[  SKIPPED ] TestMockOperationSnapshotRemoveRequest.RemoveChildError (111 ms)
...
[  PASSED  ] 14 tests.
[  SKIPPED ] 2 tests, listed below:
[  SKIPPED ] TestMockOperationSnapshotRemoveRequest.FlattenedCloneRemovesChild
[  SKIPPED ] TestMockOperationSnapshotRemoveRequest.RemoveChildError

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>