test: force source-charset to utf-8 in MSVC

prevents `tests\test-grammar-integration.cpp(483,13): error C2001: newline in constant` in non-UTF8 Windows shell

Author: MakeDecisionWorth

tests/CMakeLists.txt

@@ -1,3 +1,8 @@
+if(WIN32)
+    add_compile_options("$<$<CXX_COMPILER_ID:MSVC>:/source-charset:utf-8>")
+    add_compile_options("$<$<C_COMPILER_ID:MSVC>:/source-charset:utf-8>")
+endif()
+
 function(llama_test target)
     include(CMakeParseArguments)
     set(options)