Fix use of positional args in BUCK rules (#7760)

Summary:
Prefer to use keyword args rather than positional args for Buck rules. This appears to be the only remaining instance for `custom_unittest`

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7760

Test Plan: Search for other instances of `custom_unittest` without `name`

Reviewed By: cheng-chang

Differential Revision: D25439887

Pulled By: mzlee

fbshipit-source-id: 518c541a5c01207c7b0c1f7322addf5cc4f09f92

TARGETS

@@ -810,7 +810,7 @@ cpp_binary(
 ) if not is_opt_mode else None
 
 custom_unittest(
-    "c_test",
+    name = "c_test",
     command = [
         native.package_name() + "/buckifier/rocks_test_runner.sh",
         "$(location :{})".format("c_test_bin"),

buckifier/targets_builder.py

@@ -90,7 +90,7 @@ cpp_binary(
 ) if not is_opt_mode else None
 
 custom_unittest(
-    "c_test",
+    name = "c_test",
     command = [
         native.package_name() + "/buckifier/rocks_test_runner.sh",
         "$(location :{})".format("c_test_bin"),