A `pattern` facet's value must be a valid XSD regular expression. Two
truncation cases were compiled with a non-rejecting error and the schema was
wrongly accepted:
- a trailing `\` with no character to escape (`a\`), and
- a `[` character class the expression ends before closing with `]`
(`a[`, `[a[:xyz:`).
Both are unambiguously invalid, never an engine limitation on an otherwise
valid pattern, so they now raise the dedicated `RegexError.incompleteEscape`
and `RegexError.unterminatedClass` and `rejectsPattern` treats them as
schema-validity errors. A completed escape (`\[`, `\\`), a closed class, and a
class containing an escaped `]` or a subtraction stay valid.
Caught thirteen invalid schemas: XSTS invalid-schemas-accepted 230 -> 217, no
other bucket moved.
Mihaela Mihaljevic
50c6fb9b1c