Correct the reversed diff output for all tests

This has been driving nuts for years. The output of the diff emitted
when a test fails was always reversed, because the diff tool is called
with `${difftool} ${new} ${old}` so all the `-` and `+` contexts are
reversed, and the highlights are all screwed up.

The output of a `make test` run should show what has changed from the
baseline/expected, not how the expected differs from the actual. When
considered from both the perspective of intentional changes to the test
outputs and failed test outputs, it is desirable to see how the test
output has changed from the previously expected, and not the other way
around.

(If you were used to the previous behavior, I apologize. But it was
wrong.)

tests/interactive.fish

@@ -69,11 +69,11 @@ function test_file
         say red "fail"
         if test $out_status -ne 0
             say yellow "Output differs for file $file. Diff follows:"
-            colordiff -u $file.tmp.out $file.out
+            colordiff -u $file.out $file.tmp.out
         end
         if test $err_status -ne 0
             say yellow "Error output differs for file $file. Diff follows:"
-            colordiff -u $file.tmp.err $file.err
+            colordiff -u $file.err $file.tmp.err
         end
         if test $exit_status -ne 0
             say yellow "Exit status differs for file $file."

tests/invocation.sh

@@ -245,10 +245,10 @@ test_file() (
     mv -f "${test_stderr}.new" "${test_stderr}"
 
     # Check the results
-    if ! diff "${test_stdout}" "${want_stdout}" >/dev/null 2>/dev/null ; then
+    if ! diff "${want_stdout}" "${test_stdout}" >/dev/null 2>/dev/null ; then
         out_status=1
     fi
-    if ! diff "${test_stderr}" "${want_stderr}" >/dev/null 2>/dev/null ; then
+    if ! diff "${want_stderr}" "${test_stderr}" >/dev/null 2>/dev/null ; then
         err_status=1
     fi
 
@@ -264,11 +264,11 @@ test_file() (
 
         if [ "$out_status" != '0' ] ; then
             say "$term_yellow" "Output differs for file $file. Diff follows:"
-            "$difftool" -u "${test_stdout}" "${want_stdout}"
+            "$difftool" -u "${want_stdout}" "${test_stdout}"
         fi
         if [ "$err_status" != '0' ] ; then
             say "$term_yellow" "Error output differs for file $file. Diff follows:"
-            "$difftool" -u "${test_stderr}" "${want_stderr}"
+            "$difftool" -u  "${want_stderr}" "${test_stderr}"
         fi
         rc=1
     fi

tests/test.fish

@@ -55,11 +55,11 @@ function test_file
         say red "fail"
         if test $out_status -ne 0
             say yellow "Output differs for file $file. Diff follows:"
-            colordiff -u $base.tmp.out $base.out
+            colordiff -u $base.out $base.tmp.out
         end
         if test $err_status -ne 0
             say yellow "Error output differs for file $file. Diff follows:"
-            colordiff -u $base.tmp.err $base.err
+            colordiff -u $base.err $base.tmp.err
         end
         if test $exit_status -ne 0
             say yellow "Exit status differs for file $file."