t4015: separate common setup and per-test expectation

The last two tests in the script were to

 - set up color.diff.* slots
 - set up an expectation for a single test
 - run that test and check the result

but split in a wrong way.  It did the first two in the first test
and the third one in the second test.  The latter two belong to each
other.  This matters when you plan to add more of these tests that
share the common coloring.

While at it, make sure we use a color different from old, which is
also red.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4015-diff-whitespace.sh

@@ -810,30 +810,31 @@ test_expect_success 'setup diff colors' '
 	git config color.diff.old red &&
 	git config color.diff.new green &&
 	git config color.diff.commit yellow &&
-	git config color.diff.whitespace "normal red" &&
+	git config color.diff.whitespace blue &&
 
-	git config core.autocrlf false &&
+	git config core.autocrlf false
+'
+
+test_expect_success 'diff that introduces a line with only tabs' '
+	git config core.whitespace blank-at-eol &&
+	git reset --hard &&
+	echo "test" >x &&
+	git commit -m "initial" x &&
+	echo "{NTN}" | tr "NT" "\n\t" >>x &&
+	git -c color.diff=always diff | test_decode_color >current &&
 
-	cat >expected <<-\EOF
+	cat >expected <<-\EOF &&
 	<BOLD>diff --git a/x b/x<RESET>
 	<BOLD>index 9daeafb..2874b91 100644<RESET>
 	<BOLD>--- a/x<RESET>
 	<BOLD>+++ b/x<RESET>
 	<CYAN>@@ -1 +1,4 @@<RESET>
 	 test<RESET>
 	<GREEN>+<RESET><GREEN>{<RESET>
-	<GREEN>+<RESET><BRED>	<RESET>
+	<GREEN>+<RESET><BLUE>	<RESET>
 	<GREEN>+<RESET><GREEN>}<RESET>
 	EOF
-'
 
-test_expect_success 'diff that introduces a line with only tabs' '
-	git config core.whitespace blank-at-eol &&
-	git reset --hard &&
-	echo "test" >x &&
-	git commit -m "initial" x &&
-	echo "{NTN}" | tr "NT" "\n\t" >>x &&
-	git -c color.diff=always diff | test_decode_color >current &&
 	test_cmp expected current
 '