tmpfs tests: check for built-in tmpfs module

As of r363471, tmpfs is included in all GENERIC kernel configs. This
results in a warning being emitted for each call to kldload(8):

module_register: cannot register tmpfs from tmpfs.ko; already loaded from kernel

Check for the presence of the module via kldstat first to quiet this
warning.

Reviewed by: asomers, arichardson
Differential Revision: https://reviews.freebsd.org/D26632