t6300-for-each-ref: fix "more than one quoting style" tests

t6300-for-each-ref: fix "more than one quoting style" tests

'git for-each-ref' should error out when invoked with more than one
quoting style options.  The tests checking this have two issues:

  - They run 'git for-each-ref' upstream of a pipe, hiding its exit
    code, thus don't actually checking that 'git for-each-ref' exits
    with error code.

  - They check the error message in a rather roundabout way.

Ensure that 'git for-each-ref' exits with an error code using the
'test_must_fail' helper function, and check its error message by
grepping its saved standard error.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>