tests: net: socket: tcp: Fix build for nrf5340dk_nrf5340_cpuapp_ns

Fix build for nrf5340dk_nrf5340_cpuapp_ns.
Test takes up almost all of the flash, leaving no room for the image
to be signed. Disable the bootloader to free up flash-usage.

Fixes: #66469

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>