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/libs/Android.bp b/tests/libs/Android.bp
index 4cd991a..004f668 100644
--- a/tests/libs/Android.bp
+++ b/tests/libs/Android.bp
@@ -26,6 +26,7 @@
     sanitize: {
         never: true,
     },
+    stl: "libc++_static",
     target: {
         darwin: {
             enabled: false,