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>

migrate_cpp/cpp_refactoring/BUILD

@@ -60,6 +60,7 @@ cc_library(
 
 cc_test(
     name = "fn_inserter_test",
+    size = "small",
     srcs = ["fn_inserter_test.cpp"],
     deps = [
         ":fn_inserter",
@@ -82,6 +83,7 @@ cc_library(
 
 cc_test(
     name = "for_range_test",
+    size = "small",
     srcs = ["for_range_test.cpp"],
     deps = [
         ":for_range",
@@ -106,6 +108,7 @@ cc_library(
 
 cc_test(
     name = "var_decl_test",
+    size = "small",
     srcs = ["var_decl_test.cpp"],
     deps = [
         ":matcher_test_base",