p7821: fix test_perf invocation for prereqs

p7821: fix test_perf invocation for prereqs

Since 5dccd9155f (t/perf: add iteration setup mechanism to perf-lib,
2022-04-04), perf tests need to declare their prerequisites with
'--prereq', after the test title. p7821 was forgotten in that commit,
such that running that test on a machine where the PCRE prereq is not
satisfied aborts the test with:

    error: bug in the test script: test_wrapper_ needs 2 positional parameters

Fix this by correcting the two 'test_perf' invocations in that test
suite.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>