test: turn EXPENSIVE into a lazy prerequisite

Two test scripts (t0021 and t5551) had copy & paste code to set
EXPENSIVE prerequisite.  Use the test_lazy_prereq helper to define
them in the common t/test-lib.sh.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t0021-conversion.sh

@@ -190,8 +190,6 @@ test_expect_success 'required filter clean failure' '
 	test_must_fail git add test.fc
 '
 
-test -n "$GIT_TEST_LONG" && test_set_prereq EXPENSIVE
-
 test_expect_success EXPENSIVE 'filter large file' '
 	git config filter.largefile.smudge cat &&
 	git config filter.largefile.clean cat &&

t/t5551-http-fetch.sh

@@ -214,8 +214,6 @@ test_expect_success 'cookies stored in http.cookiefile when http.savecookies set
 	test_cmp expect_cookies.txt cookies_tail.txt
 '
 
-test -n "$GIT_TEST_LONG" && test_set_prereq EXPENSIVE
-
 test_expect_success EXPENSIVE 'create 50,000 tags in the repo' '
 	(
 	cd "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&

t/test-lib.sh

@@ -855,6 +855,10 @@ test_lazy_prereq AUTOIDENT '
 	git var GIT_AUTHOR_IDENT
 '
 
+test_lazy_prereq EXPENSIVE '
+	test -n "$GIT_TEST_LONG"
+'
+
 # When the tests are run as root, permission tests will report that
 # things are writable when they shouldn't be.
 test -w / || test_set_prereq SANITY