tests: subsys: libcxx: extend the timeout for cpp.libcxx.newlib_nano

Extend the timeout to prevent the cpp.libcxx.newlib_nano testcase runs
failed in acrn_ehl_crb, ehl_crb and up_squared. We need to give these
boards more time to finish the testcase's automation.

Fixes #36852

Signed-off-by: Enjia Mai <enjia.mai@intel.com>