test(regex): accept common valid patterns

[kabirvashisht4-glitch]

Closes #1213.

After #1210, regex.json had six invalid patterns but only one valid string, ([abc])+\s+$. A checker that rejected ordinary syntax could still pass the whole file. This adds valid cases:

instance	what it exercises
""	the empty pattern
a|b	alternation
a{2,3}	a range quantifier
a+?	a non-greedy quantifier
[^a-z]	a complemented range class

Why these five

All but "" come from the portable subset the core spec asks schema authors to limit themselves to (the list discussed in #898). Each one compiles in V8 with no flag, with u and with v, and in Python re, Ruby, Perl and Java's Pattern. python-jsonschema, jsonschema-rs and fastjsonschema accept all five as well.

Left out because an engine rejects them: lookarounds and backreferences (RE2-based engines), \p{Lu} (Python re), a{65536} (Perl) and [a-] (V8 with v).

Scope

draft3, draft7, draft2019-09, draft2020-12 and v1, the five drafts that have regex.json. The copies are identical apart from the schema object.

Verification

bin/jsonschema_suite check passes 11/11 with no skips (jsonschema==4.19.0).

Run against the edited draft2020-12 file (18 cases, 12 in draft3):

checker	before	after
toy checker that rejects "", |, {, +? and [^	13/13	13/18
python-jsonschema 4.19.0 regex (re.compile)	13/13	18/18
V8 new RegExp(s, "u")	13/13	18/18

No engine I tried fails any added case, so like #1210 this is regression coverage, not a fix for a known-failing implementation. No open PR touches regex.json.