ART: Allow scoped adjustments to log verbosity

Add ScopedLogSeverity to adjust the logging level.

Suppress warnings by default in gtests. Suppress errors in instances
where errors are expected.

Change-Id: If3ef865813e9505ab60bc90baed63ff11d90afbb
diff --git a/runtime/gc/space/space_test.h b/runtime/gc/space/space_test.h
index 9f39b80..09d10dd 100644
--- a/runtime/gc/space/space_test.h
+++ b/runtime/gc/space/space_test.h
@@ -127,6 +127,9 @@
 }
 
 void SpaceTest::InitTestBody(CreateSpaceFn create_space) {
+  // This will lead to error messages in the log.
+  ScopedLogSeverity sls(LogSeverity::FATAL);
+
   {
     // Init < max == growth
     std::unique_ptr<Space> space(create_space("test", 16 * MB, 32 * MB, 32 * MB, nullptr));