libsnapshot: Fix flaky low-space tests.

These tests have been a plague of flakiness forever, and while we've
made improvements, the test is fundamentally flaky, and keeps showing up
in presubmit.

This change removes the code that attempts to fill /data until it's
almost full, which is unreliable and destructive. Instead, attempt to
allocate a file that is one block smaller than the maximum file size.
This should always fail with ENOSPC, as opposed to the maximum file size
which would fail with EFBIG.

Bug: N/A
Test: vts_libsnapshot_test
Change-Id: I4652b83e31c50975ddb50b6cbe846e102ba0f322