pactest: check for pacman binary before running

Using fakeroot or fakechroot as the command with subprocess.call
prevents the detection and reporting of a missing pacman binary.  Some
tests even pass when run with a non-existent binary.  Checking manually
allows us to provide a meaningful error message and prevent the false
positives.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>

test/pacman/pactest.py

@@ -49,7 +49,7 @@ def create_parser():
                       help = "set debug level for pacman")
     parser.add_option("-p", "--pacman", action = "callback",
                       callback = resolve_binary_path, type = "string",
-                      dest = "bin", default = "pacman",
+                      dest = "bin", default = util.which("pacman"),
                       help = "specify location of the pacman binary")
     parser.add_option("--keep-root", action = "store_true",
                       dest = "keeproot", default = False,
@@ -86,6 +86,10 @@ def create_parser():
     opt_parser = create_parser()
     (opts, args) = opt_parser.parse_args()
 
+    if opts.bin is None or not os.access(opts.bin, os.X_OK):
+        tap.bail("cannot locate pacman binary")
+        sys.exit(2)
+
     # instantiate env
     root_path = tempfile.mkdtemp(prefix='pactest-')
     env = pmenv.pmenv(root=root_path)