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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

t/t0021-conversion.sh

@@ -20,8 +20,7 @@ EOF
 generate_random_characters () {
 	LEN=$1
 	NAME=$2
-	test-tool genrandom some-seed $LEN |
-		perl -pe "s/./chr((ord($&) % 26) + ord('a'))/sge" >"$TEST_ROOT/$NAME"
+	test-tool genrandom some-seed | tr -dc 'a-z' | test_copy_bytes "$LEN" >"$TEST_ROOT/$NAME"
 }
 
 filter_git () {
@@ -619,7 +618,7 @@ test_expect_success 'required process filter should be used only for "clean" ope
 	)
 '
 
-test_expect_success PERL_TEST_HELPERS 'required process filter should process multiple packets' '
+test_expect_success 'required process filter should process multiple packets' '
 	test_config_global filter.protocol.process "test-tool rot13-filter --log=debug.log clean smudge" &&
 	test_config_global filter.protocol.required true &&
 
@@ -684,7 +683,7 @@ test_expect_success PERL_TEST_HELPERS 'required process filter should process mu
 	)
 '
 
-test_expect_success PERL_TEST_HELPERS 'required process filter with clean error should fail' '
+test_expect_success 'required process filter with clean error should fail' '
 	test_config_global filter.protocol.process "test-tool rot13-filter --log=debug.log clean smudge" &&
 	test_config_global filter.protocol.required true &&
 	rm -rf repo &&