t6300: fix issues related to %(contents:size)

t6300: fix issues related to %(contents:size)

b6839fda68 (ref-filter: add support for %(contents:size), 2020-07-16)
added a new format for ref-filter, and added a function to generate
tests for this new feature in t6300.  Unfortunately, it tries to run
`test_expect_sucess' instead of `test_expect_success', and writes
$expect to `expected', but tries to read `expect'.  Those two issues
were probably unnoticed because the script only printed errors, but did
not crash.  This fixes these issues.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>