test: make vinyl/replica_rejoin more stable

The test checks that files left after rebootstrap are removed by the
garbage collector. It does that by printing file names to the result
file. This is inherently unstable, because should timing change, and
we can easily get an extra dump or compaction resulting in a different
set of files and hence test failure. Let's rewrite the test so that
it checks that files are actually removed using fio.path.exists().