tests: drivers: modem: Specify targets for cellular

Many boards do not build properly if UART_ASYNC_API
is enabled, which is causing an overly large list
of targets to exclude for the
build_all.modem.modem_cellular.build test suite.

This commit instead specifies 4 emulated boards, two
of 32-bit, two of 64-bit instead.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>