vfs: fix rename tests by waiting for writes to complete

Before this change the background writing of the file was racing with
the test of the object on the remote.

This meant that the tests passed locally but failed on a lot of the
remotes.

bin/test-repeat-vfs.sh

@@ -16,10 +16,6 @@ vfs/vfscache/downloaders
 cmd/cmount
 "
 
-testdirs="
-cmd/cmount
-"
-
 for testdir in ${testdirs}; do
     echo "Testing ${testdir}"
     cd ${base}/${testdir}

vfs/file_test.go

@@ -265,9 +265,12 @@ func testFileRename(t *testing.T, mode vfscommon.CacheMode, inCache bool, forceC
 		require.NoError(t, err)
 		require.Equal(t, 14, n)
 
+		require.NoError(t, file.SetModTime(item.ModTime))
+
 		err = fd.Close()
 		require.NoError(t, err)
 	}
+	vfs.WaitForWriters(waitForWritersDelay)
 
 	// check file in cache
 	if inCache {