[nrf fromtree] tests: net: dhcpv6: Increase net_mgmt thread stack space

The current default stack space for net_mgmt thread is 800 bytes
and it can be too small in some boards like nrf5340dk/nrf5340/cpuapp
like shown in this log. Increase the stack to 1024 so that we avoid
updating the value in near future.

Running TESTSUITE dhcpv6_tests
===================================================================
START - test_confirm_exchange_after_iface_down
E: ***** USAGE FAULT *****
E:   Stack overflow (context area not valid)
E: r0/a1:  0x00010fc4  r1/a2:  0x01000000  r2/a3:  0x00000020
E: r3/a4:  0x200022a8 r12/ip:  0x00000020 r14/lr:  0x20003088
E:  xpsr:  0x00011400
E: Faulting instruction address (r15/pc): 0x200030b8
E: >>> ZEPHYR FATAL ERROR 2: Stack overflow on CPU 0
E: Current thread: 0x20000d88 (net_mgmt)
E: Halting system

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>
(cherry picked from commit 46f59b0)
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

Author: jukkar

tests/net/dhcpv6/prj.conf

@@ -15,3 +15,4 @@ CONFIG_NET_TC_TX_COUNT=1
 CONFIG_NET_MGMT=y
 CONFIG_NET_MGMT_EVENT=y
 CONFIG_NET_MGMT_EVENT_INFO=y
+CONFIG_NET_MGMT_EVENT_STACK_SIZE=1024