[nrf fromtree] 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#zephyrproject-rtos#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>
(cherry picked from commit 5a1a1fe)
Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

Author: SgrrZhf

tests/net/lib/lwm2m/content_json/prj.conf

@@ -9,3 +9,5 @@ CONFIG_LWM2M=y
 CONFIG_LWM2M_COAP_MAX_MSG_SIZE=512
 CONFIG_LWM2M_RW_JSON_SUPPORT=y
 CONFIG_JSON_LIBRARY=y
+
+CONFIG_PICOLIBC_IO_FLOAT=y

tests/net/lib/lwm2m/content_plain_text/prj.conf

@@ -6,3 +6,5 @@ CONFIG_ZTEST_NEW_API=y
 CONFIG_ENTROPY_GENERATOR=y
 CONFIG_TEST_RANDOM_GENERATOR=y
 CONFIG_LWM2M=y
+
+CONFIG_PICOLIBC_IO_FLOAT=y