tests/9p: remove unnecessary g_strdup() calls

This is a leftover from before the recent function merge and
refactoring patches:

As these functions do not return control to the caller in
between, it is not necessary to duplicate strings passed to them.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <0f80141cde3904ed0591354059da49d1d60bcdbc.1664917004.git.qemu_oss@crudebyte.com>