Pass all check files to littlecheck at once

Instead of invoking littlecheck.py independently for each file, pass
all files at once. This amortizes the Python startup cost, and reduces
the total test time by ~15 seconds (!).

tests/test.fish

@@ -71,39 +71,26 @@ end
 
 set -g python (__fish_anypython)
 
-function test_littlecheck_file
-    set -l file $argv[1]
-    echo -n "Testing file $file ... "
-    set starttime (timestamp)
+# Test classic '.in' files.
+set -l failed 0
+for i in (string match '*.in' -- $files_to_test)
+        if not test_in_file $i
+            set failed (math $failed + 1)
+        end
+end
+
+# Test littlecheck files.
+set littlecheck_files (string match '*.fish' -- $files_to_test)
+if set -q littlecheck_files[1]
     $python -S ../littlecheck.py \
+        --progress \
         -s fish=../test/root/bin/fish \
         -s fish_test_helper=../test/root/bin/fish_test_helper \
-        $file
-    set -l exit_status $status
-    set -l res ok
-    set test_duration (delta $starttime)
-    if test $exit_status -eq 0
-        say green "ok ($test_duration $unit)"
-    end
-    return $exit_status
+        $littlecheck_files
+    set -l littlecheck_failures $status
+    set failed (math $failed + $littlecheck_failures)
 end
 
-set -l failed
-for i in $files_to_test
-    if string match --quiet '*.fish' $i
-        if not test_littlecheck_file $i
-            # littlecheck test
-            set failed $failed $i
-        end
-    else
-        # .in test
-        if not test_in_file $i
-            set failed $failed $i
-        end
-    end
-end
-
-set failed (count $failed)
 if test $failed -eq 0
     say green "All tests completed successfully"
     exit 0