Fuzzer started from second instead of first pass as default when using Driver

[matteB10]

In the run() method in driver.h, the "test_start_pass" for fuzzing was previously selected as test_start_pass = pass_names.at(1); which gives the second pass after the parser but compensates for the initial pass being the pass at start_index - 1 in the method test() in the Fuzzer class.

However, the start_index for the returned Fuzzer object returned from reader.pass_index() compensates in the other direction and adds +1 to the pass index, meaning the first pass in the pass sequence in the reader does not get tested if not given explicitly.

  return Fuzzer(reader)
           ...
          .start_index(reader.pass_index(test_start_pass))
          ...
          .test();

I just added the minimal fix to start fuzz testing from the first pass as default, but perhaps the logic in pass_index and test() could also be changed.