Fix Fysetc S6 FLASH_PAGE_SIZE and test build (#16560)

.github/workflows/test-builds.yml

         - BIGTREE_SKR_PRO
         - mks_robin
         - ARMED
+        - FYSETC_S6
 
         # Put lengthy tests last
 

buildroot/share/PlatformIO/scripts/fysetc_STM32S6.py

 from os.path import join
-Import("env", "projenv")
+Import("env")
 
 import os,shutil
 from SCons.Script import DefaultEnvironment

buildroot/share/PlatformIO/variants/FYSETC_S6/variant.h

 /* HAL configuration */
 #define HSE_VALUE               12000000U
 
-#define FLASH_PAGE_SIZE         uint32(4 * 1024)
+#define FLASH_PAGE_SIZE         (4U * 1024U)
 
 #ifdef __cplusplus
 } // extern "C"

buildroot/share/git/mftest

      f1)  TESTENV='STM32F103RE' ;;
      f4)  TESTENV='STM32F4' ;;
      f7)  TESTENV='STM32F7' ;;
+     s6)  TESTENV='FYSETC_S6' ;;
   teensy) TESTENV='teensy31' ;;
      t31) TESTENV='teensy31' ;;
      t32) TESTENV='teensy31' ;;

buildroot/share/tests/FYSETC_S6-tests

+#!/usr/bin/env bash
+#
+# Build tests for FYSETC_S6
+#
+
+# exit on first failure
+set -e
+
+# Build examples
+restore_configs
+use_example_configs FYSETC/S6
+exec_test $1 $2 "FYSETC S6 Example"
+
+# cleanup
+restore_configs

platformio.ini

   -DVECT_TAB_OFFSET=0x10000
   -DUSBCON -DUSBD_USE_CDC -DHAL_PCD_MODULE_ENABLED -DUSBD_VID=0x0483 '-DUSB_PRODUCT="FYSETC_S6"'
 build_unflags     = -std=gnu++11
-extra_scripts     = buildroot/share/PlatformIO/scripts/fysetc_STM32S6.py
+extra_scripts     = pre:buildroot/share/PlatformIO/scripts/fysetc_STM32S6.py
 src_filter        = ${common.default_src_filter} +<src/HAL/HAL_STM32>
 lib_ignore        = Arduino-L6470
 debug_tool        = stlink