tests: bluetooth: tester: Increase BT RX thread stack

Default stack size if not enought and was causing MPU faults.
This was affecting GATT/SR/GAW/BV-14-C qualification test case.

<dbg> bttester_gatt: gatt_db_add: handle 0x0000
<err> os: ***** MPU FAULT *****
<err> os:   Stacking error (context area might be not valid)
<err> os:   Data Access Violation
<err> os:   MMFAR Address: 0x2000aa38
<err> os: r0/a1:  0x00000000  r1/a2:  0x00000000  r2/a3:  0x00000000
<err> os: r3/a4:  0x00000000 r12/ip:  0x00000000 r14/lr:  0x00000000
<err> os:  xpsr:  0x00000000
<err> os: Faulting instruction address (r15/pc): 0x00000000
<err> os: >>> ZEPHYR FATAL ERROR 2: Stack overflow on CPU 0
<err> os: Current thread: 0x20002d28 (BT RX)
<err> os: Halting system

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>