libsnapshot: Improve low space tests.

These tests have historically been pretty flaky, and have gone through a
few rounds of improvement. This is yet another: remove the
parameterization, so we can test that at least one variant of the test
succeeds, rather than all of them.

Filesystems (especially F2FS) have a lot going on underneath the hood,
and there's no guarantee that precise free space can be allocated,
measured, or relied upon in the ways this test expects. And all that we
really need to test is that some kind of out-of-space error can be
triggered.

Bug: 266645706
Test: vts_libsnapshot_test
Change-Id: I9620e5b496d5020b21cc37074e87dd21fc419ed2