[uart] Fix RTL87xx compilation failure due to SUCCESS macro collision

The Realtek SDK defines `#define SUCCESS 0` in basic_types.h which
collides with the FlushResult::SUCCESS enum value, breaking compilation
on RTL87xx devices when api/wifi components pull in the SDK headers.

Add `#undef SUCCESS` before the enum definition, following the same
pattern used elsewhere in the codebase for vendor SDK macro collisions.

Also add RTL87xx UART compilation test.

tests/components/uart/test.rtl87xx-ard.yaml

@@ -0,0 +1,14 @@
+uart:
+  - id: uart_id
+    tx_pin: PA23
+    rx_pin: PA18
+    baud_rate: 9600
+    data_bits: 8
+    parity: NONE
+    stop_bits: 1
+
+switch:
+  - platform: uart
+    name: "UART Switch"
+    uart_id: uart_id
+    data: [0x01, 0x02, 0x03]