btrfs-progs: tests: Avoid debug log populating stdout

When running misc-test/034, we got unexpected log output:
      [TEST/misc]   033-filename-length-limit
      [TEST/misc]   034-metadata-uuid
  Checking btrfstune logic
  Checking dump-super output
  Checking output after fsid change
  Checking for incompat textual representation
  Checking setting fsid back to original
  Testing btrfs-image restore

This is caused by commit 2570cff076 ("btrfs-progs: test: cleanup misc-tests/034")
which uses _log facility which also populates stdout.

Just change _log() to echo "$*" >> "$RESULTS" to fix it.
Unlike the initial commit, there is no other user of _log, so it
shouldn't affect other tests.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

tests/common

@@ -51,7 +51,7 @@ _fail()
 # log a message to the results file
 _log()
 {
-	echo "$*" | tee -a "$RESULTS"
+	echo "$*" >> "$RESULTS"
 }
 
 # copy stdout to log and pass to stdout, eg. another stdout consumer, commands