more usages in f*/

Summary:
In this diff we replace tags by labels to have a single unified terminology across fbcode_macro layer.
More context and examples in https://fb.workplace.com/groups/devx.build.bffs/permalink/6292799467421839/

Differential Revision: D67127823

fbshipit-source-id: 7e809e3bad5734ca3a23ae5349996dfdc4ddfb88

folly/gen/test/BUCK

@@ -21,7 +21,7 @@ cpp_unittest(
     name = "base_test",
     srcs = ["BaseTest.cpp"],
     headers = [],
-    tags = [
+    labels = [
         "serialize_test_cases",
     ],
     deps = [
@@ -106,7 +106,7 @@ cpp_unittest(
     name = "parallel_test",
     srcs = ["ParallelTest.cpp"],
     headers = [],
-    tags = [
+    labels = [
         "serialize",
     ],
     deps = [