Fix SKRmini test required serial port(s) (#15117)

.travis.yml

   - TEST_PLATFORM="sanguino_atmega1284p"
   - TEST_PLATFORM="sanguino_atmega644p"
   - TEST_PLATFORM="STM32F1"
-  #- TEST_PLATFORM="BIGTREE_SKR_MINI"
+  - TEST_PLATFORM="BIGTREE_SKR_MINI"
   #- TEST_PLATFORM="fysetc_STM32F1"
   - TEST_PLATFORM="alfawise_U20"
   #- TEST_PLATFORM="STM32F4"

buildroot/share/tests/BIGTREE_SKR_MINI-tests

 #!/usr/bin/env bash
 #
-# Build tests for STM32F1 Bigtreetech
+# Build tests for STM32F1 Bigtreetech (SKR Mini v1.1)
 #
 
 # exit on first failure
 # Build with the default configurations
 #
 restore_configs
-opt_set MOTHERBOARD BOARD_BIGTREE_SKR_E3_DIP
-exec_test $1 $2 "Default Configuration"
+opt_set MOTHERBOARD BOARD_BIGTREE_SKR_MINI_V1_1
+opt_set SERIAL_PORT 1
+opt_set SERIAL_PORT_2 -1
+exec_test $1 $2 "Bigtreetech SKR Mini v1.1 - Basic Configuration"
 
 # clean up
 restore_configs