libsnapshot_test: skip global setup on non-VAB devices.

On non-VAB devices, fake super image creation may fail because it
may not support f2fs file pinning.

Also changes global setup to a testing::Environment object because it is
the recommended way in gtest's guide. SnapshotTestEnvironement::TearDown
is automatically executed before RUN_ALL_TEST returns.

Test: vts_libsnapshot_test
Test: make IsVirtualAbEnabled() return false, then all tests are
skipped.

Bug: 162557082
Change-Id: I1382f9a4ddec146fa2d8cfb21ac66ca22a2e110f
(cherry picked from commit a7c00a09)
Merged-In: I1382f9a4ddec146fa2d8cfb21ac66ca22a2e110f