Add session for gRPC samples. (GoogleCloudPlatform#385)

These samples are being excluded from the plain `tests` target, since
they require the gRPC libraries (which are not compatible with Python
3, yet). This means they are not running on Jenkins or Travis.

By adding another session, we will be able to run the tests for these
samples on Jenkins.

Author: tswast

nox.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import fnmatch
+import itertools
 import os
 import subprocess
 import tempfile
@@ -103,30 +104,26 @@ def setup_appengine(session):
 
 def run_tests_in_sesssion(
         session, interpreter, use_appengine=False, skip_flaky=False,
-        changed_only=False):
+        changed_only=False, sample_directories=None):
     session.interpreter = interpreter
     session.install(REPO_TOOLS_REQ)
     session.install('-r', 'requirements-{}-dev.txt'.format(interpreter))
 
     if use_appengine:
         setup_appengine(session)
-        sample_root = 'appengine/standard'
-    else:
-        sample_root = '.'
 
     pytest_args = COMMON_PYTEST_ARGS[:]
 
     if skip_flaky:
         pytest_args.append('-m not slow and not flaky')
 
-    # session.posargs is any leftover arguments from the command line, which
-    # allows users to run a particular test instead of all of them.
-    if session.posargs:
-        sample_directories = session.posargs
-    else:
-        sample_directories = collect_sample_dirs(
-            sample_root,
-            TESTS_BLACKLIST if not use_appengine else APPENGINE_BLACKLIST)
+    if sample_directories is None:
+        # session.posargs is any leftover arguments from the command line,
+        # which allows users to run a particular test instead of all of them.
+        if session.posargs:
+            sample_directories = session.posargs
+        else:
+            sample_directories = collect_sample_dirs('.', TESTS_BLACKLIST)
 
     if changed_only:
         changed_files = get_changed_files()
@@ -154,7 +151,19 @@ def session_tests(session, interpreter):
 
 def session_gae(session):
     run_tests_in_sesssion(
-        session, 'python2.7', use_appengine=True)
+        session, 'python2.7', use_appengine=True,
+        sample_directories=collect_sample_dirs(
+            'appengine/standard',
+            APPENGINE_BLACKLIST))
+
+
+def session_grpc(session):
+    run_tests_in_sesssion(
+        session,
+        'python2.7',
+        sample_directories=itertools.chain(
+            collect_sample_dirs('speech'),
+            collect_sample_dirs('bigtable')))
 
 
 @nox.parametrize('subsession', ['gae', 'tests'])
@@ -166,7 +175,10 @@ def session_travis(session, subsession):
     else:
         run_tests_in_sesssion(
             session, 'python2.7', use_appengine=True, skip_flaky=True,
-            changed_only=True)
+            changed_only=True,
+            sample_directories=collect_sample_dirs(
+                'appengine/standard',
+                APPENGINE_BLACKLIST))
 
 
 def session_lint(session):