Add some additional test cases to NetBSD's strcpy tests.
These were required to catch some bugs in the new stpcpy routine
presented in D41349.

Also extend the tests to permit loading an external strcpy function
and testing it over using the libc version. This was added by the
example of other tests in the test suite doing the same thing and
helped tremendously in development.

These bits were originally part of D41349 but were taken out by
@ngie's request.