Build bionic/tests with cpp_std experimental.

Test: bionic unit tests still pass.
Change-Id: I1ff5c0fa391ae111ac19e863ef0f7f29868406e6
diff --git a/tests/Android.bp b/tests/Android.bp
index 5483b6a..ec90296 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -17,6 +17,7 @@
 cc_defaults {
     name: "bionic_tests_defaults",
     host_supported: true,
+    cpp_std: "experimental",
     target: {
         darwin: {
             enabled: false,