fix(platform,core,test): fix Windows compatibility issues

- platform_mkdir now checks if directory exists before creating
- Add FILE_ATTRIBUTE_HIDDEN for dot-prefixed directories on Windows
- Create parent mods directory before temp directory
- Move module unload before copy in reload sequence to fix issue on Windows
- Force CMD shell in test makefile on Windows

Test makefile now uses cmd.exe instead of bash on Windows, fixing
syntax errors when running via build.bat.

test/gmake.mk

@@ -3,6 +3,8 @@ CFLAGS = -Wall -Wpedantic -I../include -std=c89
 LDFLAGS = -L../bin/debug -lstk
 
 ifeq ($(OS),Windows_NT)
+    SHELL := cmd.exe
+    .SHELLFLAGS := /c
     MODULE_EXT = .dll
     EXE_EXT = .exe
 else