Tweak lib/dns/tests/tkey_test requirements

Tweak lib/dns/tests/tkey_test requirements

Currently, the lib/dns/tests/tkey_test unit test is only run when the
linker supports the --wrap option.  However, linker support for that
option is only needed for static builds.  As a result, the unit test
mentioned before is not being run everywhere it can be run as even for
builds done using --with-libtool, the test is not run unless the linker
supports the --wrap option.

Tweak preprocessor directives in lib/dns/tests/tkey_test.c so that this
test is run:

  - for all builds using --with-libtool,
  - for static builds done using a linker supporting the --wrap option.