Correct a race condition in pipeline-pgroup test

This test launches two background processes and is sensitive to
interleaving of output. Fix it so that newlines are not output by
the background process.

Hopefully this fixes the flakiness of this test.

tests/checks/pipeline-pgroup.fish

@@ -43,7 +43,8 @@ or echo "Pgroups disagreed. Found $a0 $a1 $a2, and $b0 $b1 $b2"
 status job-control full
 $fth print_pgrp | begin
     tr \n ' '
-    $fth print_pgrp &
+    $fth print_pgrp | tr \n ' ' &
 end &
 wait
+echo
 # CHECK: {{(\d+) (?!\1)\d+}}