tests: split out snippets builds (#219)

@tmatsuo Emulating PR #207. I don't know if I'm missing anything:  e.g., I don't quite understand what the `split_system_tests=True` does in the `synth.py` there.

Toward #191

.kokoro/presubmit/presubmit.cfg

@@ -5,3 +5,7 @@ env_vars: {
     key: "RUN_SYSTEM_TESTS"
     value: "false"
 }
+env_vars: {
+    key: "RUN_SNIPPETS_TESTS"
+    value: "false"
+}

.kokoro/presubmit/snippets-2.7.cfg

@@ -0,0 +1,7 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Only run this nox session.
+env_vars: {
+    key: "NOX_SESSION"
+    value: "snippets-2.7"
+}

.kokoro/presubmit/snippets-3.8.cfg

@@ -0,0 +1,7 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Only run this nox session.
+env_vars: {
+    key: "NOX_SESSION"
+    value: "snippets-3.8"
+}

noxfile.py

@@ -112,14 +112,18 @@ def system(session):
 def snippets(session):
     """Run the snippets test suite."""
 
-    constraints_path = str(
-        CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
-    )
+    # Check the value of `RUN_SNIPPETS_TESTS` env var. It defaults to true.
+    if os.environ.get("RUN_SNIPPETS_TESTS", "true") == "false":
+        session.skip("RUN_SNIPPETS_TESTS is set to false, skipping")
 
     # Sanity check: Only run snippets tests if the environment variable is set.
     if not os.environ.get("GOOGLE_APPLICATION_CREDENTIALS", ""):
         session.skip("Credentials must be set via environment variable.")
 
+    constraints_path = str(
+        CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
+    )
+
     # Install all test dependencies, then install local packages in place.
     session.install("mock", "pytest", "google-cloud-testutils", "-c", constraints_path)
     session.install("google-cloud-storage", "-c", constraints_path)