[scudo/standalone] Don't define test main function for Fuchsia

Fuchsia's unit test library provides the main function by default.

Reviewed By: cryptoad

Differential Revision: https://reviews.llvm.org/D87809

compiler-rt/lib/scudo/standalone/tests/scudo_unit_test_main.cpp

@@ -29,11 +29,11 @@ __scudo_default_options() {
          "dealloc_type_mismatch=" DEALLOC_TYPE_MISMATCH;
 }
 
-int main(int argc, char **argv) {
+// The zxtest library provides a default main function that does the same thing
+// for Fuchsia builds.
 #if !SCUDO_FUCHSIA
+int main(int argc, char **argv) {
   testing::InitGoogleTest(&argc, argv);
   return RUN_ALL_TESTS();
-#else
-  return RUN_ALL_TESTS(argc, argv);
-#endif
 }
+#endif