Fix setting of PATH for `make check` on windows

mk/tests.mk

@@ -95,7 +95,8 @@ ifdef CFG_WINDOWSY_$(1)
                $$(if $$(findstring stage3,$$(1)), \
                     stage3/$$(CFG_LIBDIR_RELATIVE), \
                )))))/rustlib/$$(CFG_BUILD)/lib
-  CFG_RUN_TEST_$(1)=$$(call CFG_RUN_$(1),$$(call CFG_TESTLIB_$(1),$$(1),$$(4)),$$(1))
+  CFG_RUN_TEST_$(1)=$$(TARGET_RPATH_VAR$$(2)_T_$$(3)_H_$$(4)) \
+	  $$(call CFG_RUN_$(1),$$(call CFG_TESTLIB_$(1),$$(1),$$(4)),$$(1))
 endif
 
 # Run the compiletest runner itself under valgrind