Fix bionic-unit-tests-glibc

Also add another test for dlsym(RTLD_NEXT, ..)

Bug: http://b/33106624
Test: run bionic-unit-tests-glibc and bionic-unit-tests
Change-Id: I340165d66bf2360b0e3273d3561a08cb5e7bd025
diff --git a/tests/gtest_globals_cts.cpp b/tests/gtest_globals_cts.cpp
index bf891a1..2532ef1 100644
--- a/tests/gtest_globals_cts.cpp
+++ b/tests/gtest_globals_cts.cpp
@@ -18,4 +18,8 @@
 
 #include <string>
 
-const std::string g_testlib_root = "/data/local/tmp/lib/bionic-loader-test-libs";
+static const std::string g_testlib_root = "/data/local/tmp/lib/bionic-loader-test-libs";
+
+const std::string& get_testlib_root() {
+  return g_testlib_root;
+}