Auto merge of #27553 - Diggsey:win-path-fix, r=alexcrichton

I have no idea how bors keeps working without this - I can only assume it's some peculiarity of how windows searches for DLLs.

Without this change, running `make check` on windows will not correctly set PATH to include eg. `x86_64-pc-windows-gnu\stage1\bin\rustlib\x86_64-pc-windows-gnu\lib`, and when it tries to run eg. `stage1/test/stdtest-x86_64-pc-windows-gnu.exe`, it will fail because windows can't find the DLLs on which it relies.

It seems to be just a mistake: when the equivalent was added for the branch that deals with unix-like platforms, the windows branch was left unchanged.

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