tests: Fix regression test to work with new debug messages from guestfsd.

Commit 0b631d739b changed the debug
message format from the main loop in guestfsd.  This test tried to
filter out these messages when comparing output, but that stopped
working.  The problem was only evident when enabling debugging while
running the tests.

tests/regressions/rhbz557655.sh

@@ -79,7 +79,7 @@ EOF
 mv rhbz557655.err rhbz557655.err~
 grep -E 'set[-_]memsize|truncate[-_]size' rhbz557655.err~ |
   grep -Ev 'libguestfs: trace:' |
-  grep -Ev 'proc 200' > rhbz557655.err
+  grep -Ev 'guestfsd: (<=|=>) truncate_size' > rhbz557655.err
 rm rhbz557655.err~
 
 diff -u $srcdir/rhbz557655-expected.stdout rhbz557655.out