t0021: refactor `generate_random_characters()` to not depend on Perl

t0021: refactor `generate_random_characters()` to not depend on Perl

The `generate_random_characters()` helper function generates N
random characters in the range 'a-z' and writes them into a file. The
logic currently uses Perl, but it can be adapted rather easily by:

  - Making `test-tool genrandom` generate an infinite stream.

  - Using `tr -dc` to strip all characters which aren't in the range of
    'a-z'.

  - Using `test_copy_bytes()` to copy the first N bytes.

This allows us to drop the PERL_TEST_HELPERS prerequisite.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>