Prior to this, test failures caused by errors in these helpers printed
out long exec.ExitErrors structs that didn't contain any useful info,
for example:
```
Expected error:
      <*exec.ExitError | 0xc20804e518>: {
          ProcessState: {
              pid: 22855,
              status: 256,
              rusage: {
                  Utime: {Sec: 0, Usec: 312000},
                  Stime: {Sec: 0, Usec: 96000},
                  Maxrss: 42912,
                  Ixrss: 0,
                  Idrss: 0,
                  Isrss: 0,
                  Minflt: 21667,
                  Majflt: 0,
                  Nswap: 0,
                  Inblock: 0,
                  Oublock: 40,
                  Msgsnd: 0,
                  Msgrcv: 0,
                  Nsignals: 0,
                  Nvcsw: 52,
                  Nivcsw: 34,
              },
          },
      }
      exit status 1
  not to have occurred
```