Enable metaval to handle multiple wrapped tools
Enable metaval to handle multiple wrapped tools

Before, we aborted if two different verifier backends were used in
the same benchmark definition xml file. Now we determine which
verifier backend was used by looking at the first lines of output
from metaval instead.

sosy-lab/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,dbeyer/benchexec,dbeyer/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec