[Reg] Make status/unsat-core detection more robust (#1775)

[Reg] Make status/unsat-core detection more robust (#1775)

This commit changes the regression script to look for status patterns or
get-unsat-core/get-unsat-assumption commands in the benchmark file
instead of the metadata file (i.e. the .expect file if it exists or the
benchmark file otherwise). This fix is not strictly necessary for our
current regressions but might have lead to surprising outcomes in the
future.