tests: build_all: modem: test esp-at

Add build tests for the `espressif,esp-at` driver, together with both
the interrupt and async variants of modem_ifact_uart.

Only build the async API variant for emulated platforms, as many
platforms do not build cleanly when `UART_ASYNC_API` is enabled without
custom setup of DMA nodes.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>