tests: Fix for misc/fallocate test build failure.

This was already a known possible failure case. However, in the migration to a
clearer separation of stdout and stderr in the testsuite, the logfile used to
detect the failure was not updated.

Signed-off-by: Mark Hindley <mark@hindley.org.uk>

tests/ts/misc/fallocate

@@ -30,7 +30,7 @@ else
 	# fs type of $TS_OUTDIR, could be used to skip this test early
 	fs_type=$(${TS_CMD_FINDMNT} -n -o FSTYPE -T ${TS_OUTDIR})
 
-	grep -qi "fallocate: fallocate failed:.*not supported" $TS_OUTPUT \
+	grep -qi "fallocate: fallocate failed:.*not supported" $TS_ERRLOG \
 		&& ts_skip "'${fs_type}' not supported"
 fi