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