Halve iteration count for some RefBase tests

The RacingDestructors test was occasionally timing out, by non-huge
amounts. Halve the number of iterations for long-running tests here.

Bug: 284964396
Test: Treehugger
Change-Id: If639ce98fbeb783431f07006dece4cd69f9f8b50
diff --git a/libutils/RefBase_test.cpp b/libutils/RefBase_test.cpp
index aed3b9b..d675598 100644
--- a/libutils/RefBase_test.cpp
+++ b/libutils/RefBase_test.cpp
@@ -28,7 +28,7 @@
 
 using namespace android;
 
-static constexpr int NITERS = 1000000;
+static constexpr int NITERS = 500000;
 
 static constexpr int INITIAL_STRONG_VALUE = 1 << 28;  // Mirroring RefBase definition.