meson: build qapi tests library

- builds QAPI builtins types/visitor to fix a linking issue with
  unresolved symbols in the static library.

- work around a meson limitation on generated file output directories.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200828110734.1638685-2-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>