qt: Don't use qstrdup in test passphrase cb

* lang/qt/tests/t-support.h (TestPassphraseProvider::getPassphrase):
Use gpgrt_asprintf instead of strdup.

To avoid problems on MacOS we want to avoid strdup so that
qgpgme can be built without extensions. But qstrdup allocates
with new and not with malloc, so use gpgrt_asprintf instead.