Add back test-plugins and, after making sure it is always defined,
do so unconditionally.

Reported-by: Alex Bennée <alex.bennee@linaro.org>
Fixes: 2c13c574 ("configure, meson: move --enable-plugins to meson", 2023-09-07)
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>