Add size="small" to test targets where advised (#3326)

Running `bazel test //...` reported:

```
Test execution time outside of range for MODERATE tests.
Consider setting timeout="short" or size="small".
```

This change adds size="small" to avoid such warnings being reported.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

github_tools/BUILD

@@ -13,6 +13,7 @@ py_library(
 
 py_test(
     name = "github_helpers_test",
+    size = "small",
     srcs = ["github_helpers_test.py"],
     python_version = "PY3",
     deps = [":github_helpers"],
@@ -27,6 +28,7 @@ py_binary(
 
 py_test(
     name = "pr_comments_test",
+    size = "small",
     srcs = ["pr_comments_test.py"],
     python_version = "PY3",
     deps = [":pr_comments"],