Switch bionic-unit-tests to libc++_static.

Bug: http://b/27549327
Test: run bionic-unit-tests, bionic-unit-tests-glibc.
Change-Id: I719899624dec83728a3d5b8169079e5c91c63c04
diff --git a/tests/Android.bp b/tests/Android.bp
index 84be7bc..fe34b9f 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -366,6 +366,8 @@
 
     include_dirs: ["bionic/libc"],
 
+    stl: "libc++_static",
+
     target: {
         android: {
             shared_libs: [