Add test mapping to run the libunwind tests.

Created a special target, libunwindstack_unit_test, that doesn't
include the test that dlopen's a shared library. It appears atest
doesn't understand how to handle the require keyword.

Also, move the shared library into the libunwindstack_test directory
itself.

Test: Ran atest libunwindstack_unit_test.
Change-Id: I967919b1d74a08669b61d0363d80861685725609
diff --git a/libunwindstack/TEST_MAPPING b/libunwindstack/TEST_MAPPING
new file mode 100644
index 0000000..55771c0
--- /dev/null
+++ b/libunwindstack/TEST_MAPPING
@@ -0,0 +1,7 @@
+{
+  "presubmit": [
+    {
+      "name": "libunwindstack_unit_test"
+    }
+  ]
+}