t5562: do not reuse output files

Some expected failures of git-http-backend leaves running its children
(receive-pack or upload-pack) which still hold opened descriptors
to act.err and with some probability they live long enough to write
there their failure messages after next test has already truncated
the files. This causes occasional failures of the test script.

Avoid the issue by using separated output and error file for each test,
apprending the test number to their name.
Reported-by: NCarlo Arenas <carenas@gmail.com>
Helped-by: NCarlo Arenas <carenas@gmail.com>
Helped-by: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NMax Kirillov <max@max630.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t5562-http-backend-content-length.sh

@@ -8,12 +8,12 @@ test_lazy_prereq GZIP 'gzip --version'
 verify_http_result() {
 	# some fatal errors still produce status 200
 	# so check if there is the error message
-	if grep 'fatal:' act.err
+	if grep 'fatal:' act.err.$test_count
 	then
 		return 1
 	fi
 
-	if ! grep "Status" act.out >act
+	if ! grep "Status" act.out.$test_count >act
 	then
 		printf "Status: 200 OK\r\n" >act
 	fi
@@ -33,7 +33,7 @@ test_http_env() {
 		REQUEST_METHOD=POST \
 		"$PERL_PATH" \
 		"$TEST_DIRECTORY"/t5562/invoke-with-content-length.pl \
-		    "$request_body" git http-backend >act.out 2>act.err
+		    "$request_body" git http-backend >act.out.$test_count 2>act.err.$test_count
 }
 
 ssize_b100dots() {
@@ -161,7 +161,7 @@ test_expect_success 'empty CONTENT_LENGTH' '
 		GIT_HTTP_EXPORT_ALL=TRUE \
 		REQUEST_METHOD=GET \
 		CONTENT_LENGTH="" \
-		git http-backend <empty_body >act.out 2>act.err &&
+		git http-backend <empty_body >act.out.$test_count 2>act.err.$test_count &&
 	verify_http_result "200 OK"
 '