tests: shell: also build UART shell for coverage

This enables building the UART shell alongside with the UART
backend test. The UART shell source file was not built in CI
before this, so this will provide build coverage.

Signed-off-by: Daniel Leung <[email protected]>

Author: dcpleung

tests/subsys/shell/shell_backend_uart/prj.conf

@@ -7,3 +7,4 @@ CONFIG_SHELL_METAKEYS=n
 CONFIG_LOG=n
 CONFIG_ZTEST=y
 CONFIG_EMUL=y
+CONFIG_UART_SHELL=y