Write all coverage information into a single file,
instead of one output file for each input file.
This avoids problems if the path of one input file
is longer than the allowed file name length,
because we had to encode the path into the file name.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13541 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c