UTS46 test data fixes for 175-A88

[markusicu]

For https://github.com/unicode-org/properties/issues/162

which contains discussion for
[175-A88] Action Item for Mark Davis, PAG: Review IdnaTestV2.txt and ensure that the documentation and output of status codes conforms to the specification; see L2/23-079 item 6.3 for a report of mismatches; for Unicode Version 15.1.

Review one commit at a time.

Changes, in the order of the bug report items:

• P4: fixed header comment
• in-browser-test-friendly: avoid the last label consisting of only ASCII digits (second commit)
• U1 not V7 if disallowed_STD3_* (third commit)
• NV8: Updated header comment, copied from issue with minor changes
• A3: Added a test case that triggers it (unpaired surrogate), and a header comment that it can be filtered out
• X3:
  • The implementation "knows about it" but it's unreachable code
  • Added a code comment
  • Updated header comment, copied from issue but simplified it
• "CheckBidi is not V8" was already fixed in an earlier PR

[markusicu]

Thanks Josh! @macchiati ok to merge?