Add %0 format to run-tests.php

This format matches against null bytes, and prevents the test
expectation from being interpreted as binary data.

bless_tests.php will automaticall

Add %0 format to run-tests.php

This format matches against null bytes, and prevents the test
expectation from being interpreted as binary data.

bless_tests.php will automatically replace \0 with %0 as well.

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Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

Extensions migrated in part 3:
* ftp
* gmp
* iconv
* opcache
* shmop

Fix literal compaction collision between string and double

For the sake of simplicity I'm using a separate hashtable, rather
than trying to do hash perturabation on the double strings.