Build nolocale-test with /utf-8 on MSVC (#4919)

nolocale-test compiles src/format.cc directly, so it does not pick up the
/utf-8 that CMakeLists.txt adds to the fmt target. On MSVC the static_assert
in base.h then fires: "Unicode support requires compiling with /utf-8".

The target only exists under FMT_PEDANTIC, which the Windows workflow does not
set, so this has not shown up in CI. unicode-test already guards the same flag
the same way.

test/CMakeLists.txt

@@ -134,6 +134,8 @@ if (FMT_PEDANTIC)
                              PRIVATE ${PROJECT_SOURCE_DIR}/include)
   target_compile_definitions(nolocale-test
                              PRIVATE FMT_STATIC_THOUSANDS_SEPARATOR=1)
+  target_compile_options(nolocale-test
+                         PRIVATE $<$<CXX_COMPILER_ID:MSVC>:/utf-8>)
 endif ()
 
 # These tests are disabled on Windows because they take too long.