Invalid regex tests

[Protryon]

Tests in pass/pass-explicit:

66e383bfd18e66ab.js
78c215fabdf13bae.js
bf49ec8d96884562.js
e4a43066905a597b.js

These tests are asserting non-conforming behavior to the spec.

66e383bfd18e66ab.js:
Illegal to backreference n when n is > nParenthesis for unicode mode.

78c215fabdf13bae.js:
Backtracking is removed for {/} characters in unicode mode.

bf49ec8d96884562.js:
Cannot repeat lookaheads in unicode mode.

e4a43066905a597b.js
Same backtracking stuff in unicode mode.

@bakkot