Demote test_utils from LOCAL_DEPS in noxfile

The test_utils dependency is only nedeed for test sessions, but not
for some other nox sessions such as "lint" and "docs".

bigquery/noxfile.py

@@ -20,11 +20,7 @@
 import nox
 
 
-LOCAL_DEPS = (
-    os.path.join("..", "api_core[grpc]"),
-    os.path.join("..", "core"),
-    os.path.join("..", "test_utils"),
-)
+LOCAL_DEPS = (os.path.join("..", "api_core[grpc]"), os.path.join("..", "core"))
 
 BLACK_PATHS = ("docs", "google", "samples", "tests", "noxfile.py", "setup.py")
 
@@ -44,6 +40,7 @@ def default(session):
 
     dev_install = ".[all]"
     session.install("-e", dev_install)
+    session.install("-e", os.path.join("..", "test_utils"))
 
     # IPython does not support Python 2 after version 5.x
     if session.python == "2.7":
@@ -89,6 +86,7 @@ def system(session):
         session.install("-e", local_dep)
     session.install("-e", os.path.join("..", "storage"))
     session.install("-e", ".[all]")
+    session.install("-e", os.path.join("..", "test_utils"))
 
     # IPython does not support Python 2 after version 5.x
     if session.python == "2.7":
@@ -116,6 +114,7 @@ def snippets(session):
         session.install("-e", local_dep)
     session.install("-e", os.path.join("..", "storage"))
     session.install("-e", ".[all]")
+    session.install("-e", os.path.join("..", "test_utils"))
 
     # Run py.test against the snippets tests.
     session.run("py.test", os.path.join("docs", "snippets.py"), *session.posargs)