build_minios: Fix arguments + add unittest

Add missing unittest for `build_minios`.
Check that the arguments are forwarded correctly.

BUG=b:191180137
TEST=./run_tests

Change-Id: If91b97b814434ea0e0be4b62f2d4c6bda7cbe0cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2979377
Tested-by: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/scripts/build_minios.py b/scripts/build_minios.py
index 9379eed..d0585ea 100644
--- a/scripts/build_minios.py
+++ b/scripts/build_minios.py
@@ -19,20 +19,20 @@
   parser = commandline.ArgumentParser(description=__doc__)
   parser.add_argument('--board', '-b', '--build-target', required=True,
                       help='The board name.')
-  parser.add_argument('--image', type='path',
+  parser.add_argument('--image', type='path', required=True,
                       help='The path to the chromium os image.')
   parser.add_argument('--keys-dir', type='path',
                       help='The path to keyset.',
                       default=constants.VBOOT_DEVKEYS_DIR)
-  parser.add_argument('--public-key', type='path',
+  parser.add_argument('--public-key',
                       help='Filename to the public key whose private part '\
                       'signed the keyblock.',
                       default=constants.RECOVERY_PUBLIC_KEY )
-  parser.add_argument('--private-key', type='path',
+  parser.add_argument('--private-key',
                       help='Filename to the private key whose public part is '\
                       'baked into the keyblock.',
                       default=constants.RECOVERY_DATA_PRIVATE_KEY )
-  parser.add_argument('--keyblock', type='path',
+  parser.add_argument('--keyblock',
                       help='Filename to the kernel keyblock.',
                       default=constants.RECOVERY_KEYBLOCK)
   parser.add_argument('--serial', type=str,