tests: net: lib: lwm2m: fix the missing float support

The tests `content_json` and `content_plain_test` depend on the float
support of libc. After PR##57340, Picolibc would be selected in
these two tests and the `PICOLIBC_IO_FLOAT` won't be selected if
the platform doesn't select `FPU`.

This commit select `CONFIG_PICOLIBC` and `CONFIG_PICOLIBC_IO_FLOAT`
for these two tests.

Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>