test, watcom makefile: change copy command to use a dos path separator

fixes copy failure with recent open watcom wmake changes. (Issue reported
mainstream as https://github.com/open-watcom/open-watcom-v2/issues/1281)

test/watcom.mif

@@ -108,14 +108,14 @@ testutils.lib: testutils.obj
 check: .SYMBOLIC $(TESTS)
   @set SDL_AUDIODRIVER=dummy
   @set SDL_VIDEODRIVER=dummy
-  @copy "../SDL2.dll" .
+  @copy ..\SDL2.dll .
   @for %exe in ($(TESTS)) do %exe
 
 check-quick: .SYMBOLIC $(TESTS)
   @set SDL_TESTS_QUICK=1
   @set SDL_AUDIODRIVER=dummy
   @set SDL_VIDEODRIVER=dummy
-  @copy "../SDL2.dll" .
+  @copy ..\SDL2.dll .
   @for %exe in ($(TESTS)) do %exe
 
 clean: .SYMBOLIC