Fix two issues in fuzzing infrastructure:
- Correct artifact upload path from promql/testdata/fuzz to util/fuzzing/testdata/fuzz to match where Go stores crash artifacts
- Fix GetCorpusForFuzzParseExpr to preserve original parser flag values instead of always resetting them to false, which was disabling experimental features before actual fuzzing ran
Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>
