drivers: serial: uart_emul: support interrupt-driven receive

Previously, the uart_emul device did not support interrupt-driven
receive, which is an unwritten requirement for hooking up a uart to
the Zephyr console driver.

The console is a fairly high-traffic subsystem, and we should be able
to perform more extensive testing on it, aside from maching twister
output against a regular expression. With this change, we can use
the emulation uart within the body of a ZTest.

Testing Done:
```shell
west build -p auto -b qemu_riscv64 -t run tests/drivers/console_switching
...
*** Booting Zephyr OS build zephyr-v3.4.0-4277-gae0d63471be1 ***
Running TESTSUITE console_switching
===================================================================
START - test_read
read "Hello, uart_emul0!" from uart_emul0
read "Hello, uart_emul1!" from uart_emul1
read "Hello, uart_emul0!" from uart_emul0
read "Hello, uart_emul1!" from uart_emul1
 PASS - test_read in 0.005 seconds
===================================================================
START - test_write
wrote "Hello, uart_emul0!" to uart_emul0
wrote "Hello, uart_emul1!" to uart_emul1
wrote "Hello, uart_emul0!" to uart_emul0
wrote "Hello, uart_emul1!" to uart_emul1
 PASS - test_write in 0.003 seconds
===================================================================
TESTSUITE console_switching succeeded

------ TESTSUITE SUMMARY START ------

SUITE PASS - 100.00% [console_switching]: pass = 2, fail = 0, skip = 0...
 - PASS - [console_switching.test_read] duration = 0.005 seconds
 - PASS - [console_switching.test_write] duration = 0.003 seconds

------ TESTSUITE SUMMARY END ------

===================================================================
PROJECT EXECUTION SUCCESSFUL
```

Signed-off-by: Christopher Friedt <cfriedt@meta.com>