tests: check for ps(1) existence.

Before:

    Traceback (most recent call last):
      ...
      File ".../python-afl/tests/test_fuzz.py", line 143, in stray_process_cleanup
        ps = ipc.Popen(['ps', 'ax'], stdout=ipc.PIPE)
      File "/usr/lib/python2.7/subprocess.py", line 394, in __init__
        errread, errwrite)
      File "/usr/lib/python2.7/subprocess.py", line 1047, in _execute_child
        raise child_exception
    OSError: [Errno 2] No such file or directory

After:

    Traceback (most recent call last):
      ...
      File ".../python-afl/tests/test_fuzz.py", line 140, in stray_process_cleanup
        require_commands('ps')
      File ".../python-afl/tests/tools.py", line 151, in require_commands
        raise RuntimeError('{cmd} not found; is {reason}?'.format(cmd=cmd, reason=reason))
    RuntimeError: ps(1) not found; is procps installed?

Author: jwilk

Diff for: tests/test_fuzz.py

@@ -137,6 +137,7 @@ def _test_fuzz(workdir, target, dumb=False):
 def stray_process_cleanup():
     # afl-fuzz doesn't always kill the target process:
     # https://groups.google.com/d/topic/afl-users/E37s4YDti7o
+    require_commands('ps')
     try:
         yield
     finally:

Diff for: tests/tools.py

@@ -143,7 +143,12 @@ def require_commands(*cmds):
             if os.access(path, os.X_OK):
                 break
         else:
-            raise RuntimeError('{cmd} not found; is PATH set correctly?'.format(cmd=cmd))
+            if cmd == 'ps':
+                cmd = 'ps(1)'
+                reason = 'procps installed'
+            else:
+                reason = 'PATH set correctly'
+            raise RuntimeError('{cmd} not found; is {reason}?'.format(cmd=cmd, reason=reason))
 
 def run(cmd, stdin='', xstatus=0):
     child = ipc.Popen(