print-failed-test-output: minor improvements

Drop the subshell used to create the string of dashes (and rename the
variable to "sep" at the same time).  Replace $(cat file) with the
equivalent but faster $(< file).

print-failed-test-output

@@ -3,11 +3,11 @@
 shopt -s failglob
 
 # Print output from failing tests
-dashes=$(printf "%80s" '' | tr ' ' '-')
+printf -v sep "%0.s-" {1..80}
 for exit_file in t/test-results/*.exit; do
-    [ "$(cat "$exit_file")" -eq 0 ] && continue
+    [ "$(< "$exit_file")" -eq 0 ] && continue
     out_file="${exit_file%exit}out"
-    printf '\n%s\n%s\n%s\n' "$dashes" "$out_file" "$dashes"
+    printf '\n%s\n%s\n%s\n' "$sep" "$out_file" "$sep"
     cat "$out_file"
 done
 exit 1