test/fuzz: speedup string serialization

- clamp before cleaning string because cleaning is not cheap
  (O(n), where max n is equal to kMaxStrLength)
- call cleaning for identifiers only, there is no sense to
  cleaning string literals
- replace symbols disallowed by Lua grammar in indentifier's
  names with '_'

The patch saves 16 sec on 145k samples (401 sec before the patch
and 385 sec after the patch). It is actually not so much, but it
is about 2.5 min per hour.

NO_CHANGELOG=testing
NO_DOC=testing