Tests: Bluetooth: Tester: enable EATT by default

Since Zephyr supports EATT, which is reflected in ICS GATT_2_3
(Enhanced ATT bearer Attribute Protocol Supported (L2CAP fixed EATT PSM
supported) and GATT_2_3a (Enhanced ATT bearer supported over LE)
in GATT tests preamble 4.2.1.4 (Setup EATT Bearer over LE) is used
instead of 4.2.1.2 (Setup ATT Bearer over LE). Let's enable
EATT by default, allowing to run almost all GATT tests without
additional overlay.

Signed-off-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>