Set up pytest

With this change, you can run py.test from the Chromite root, and it
will pass.

Lots of tests are explicitly marked as skipped via pytestmarks. We will
need to address these. Until then, py.test does not replace run_tests.

BUG=chromium:934414
TEST=Run both py.test and run_tests, both inside and outside the chroot.
Outside the chroot, try run_tests with and without pytest installed.
Outside the chroot, expect failures from lib/operation_unittest and
lib/cros_test_lib_unittest; this is consistent with ToT.
With py.test outside the chroot, need to add 'inside_only' to the
excluded markers in pytest.ini. Later, when py.test is run from a
script, or when the runtime env is standardized via venv, this will not
be necessary.

Change-Id: Iee21130b7afd519d264e813974834d301f05bb94
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1995734
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Greg Edelston <gredelston@google.com>
Tested-by: Greg Edelston <gredelston@google.com>
diff --git a/scripts/build_api_unittest.py b/scripts/build_api_unittest.py
index 2179c4a..f67e563 100644
--- a/scripts/build_api_unittest.py
+++ b/scripts/build_api_unittest.py
@@ -15,6 +15,8 @@
 from chromite.lib import osutils
 from chromite.scripts import build_api
 
+pytestmark = cros_test_lib.pytestmark_sigterm
+
 
 assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'