drivers: serial: serial_test: Move ring buf dep to Kconfig

This commit moves the dependency management between the
RING_BUFFER and UART_ASYNC_API or UART_INTERRUPT_DRIVEN
options to the Kconfig Kconfig.test.

If either UART API options listed are selected, the
RING_BUFFER option must be selected. This is now handled
automatically by Kconfig instead of causing a build
assert.

The asserts where added with this PR #59880, and are
removed in this commit.

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