[scudo] Allow using a different test main.

[cferris]

Summary

Fuchsia already uses a different main function for tests, so allow
anybody to use this mechanism.

Specifically, Android has a test main that allows tests to be run
in isolation and in parallel which speeds up the unit test runs
from ~14 seconds to ~4 seconds.

compiler-rt/lib/scudo/standalone/tests/scudo_unit_test.h

 #endif
 
 #if SCUDO_DEBUG
 #define SKIP_NO_DEBUG(T) T
 #else
 #define SKIP_NO_DEBUG(T) DISABLED_##T
 #endif
 
+#if SCUDO_FUCHSIA
+// The zxtest library provides a default main function that does the same thing
+// for Fuchsia builds.
+#define SCUDO_NO_TEST_MAIN
+#endif
+
 extern bool UseQuarantine;

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

__scudo_default_options() {
   EnableMemoryTaggingIfSupported();
   if (!UseQuarantine)
     return "dealloc_type_mismatch=" DEALLOC_TYPE_MISMATCH;
   return "quarantine_size_kb=256:thread_local_quarantine_size_kb=128:"
          "quarantine_max_chunk_size=512:"
          "dealloc_type_mismatch=" DEALLOC_TYPE_MISMATCH;
 }
 
-// The zxtest library provides a default main function that does the same thing
-// for Fuchsia builds.
-#if !SCUDO_FUCHSIA
+#if !defined(SCUDO_NO_TEST_MAIN)
 int main(int argc, char **argv) {
   EnableMemoryTaggingIfSupported();
   testing::InitGoogleTest(&argc, argv);
   return RUN_ALL_TESTS();
 }
 #endif