python/test: Faster tests and better BUILD (#9791)

bazel/python.bzl

@@ -0,0 +1,12 @@
+# py_test_module_list creates a py_test target for each 
+# Python file in `files`
+def py_test_module_list(files, size, deps, extra_srcs, **kwargs):
+  for file in files:
+    # remove .py
+    name = file[:-3]
+    native.py_test(
+      name=name,
+      size=size,
+      srcs=extra_srcs+[file],
+      **kwargs
+    )