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.
debug_zval_dump() currently prints refcount 1 for interned strings
and arrays, which does not really reflect the truth. These values
are not refcounted, so the refcount is misleading. Instead print
an "interned" tag.
Closes GH-6598.
