patch 9.1.0791: tests: errors in gen_opt_test.vim are not shown

Problem:  tests: errors in gen_opt_test.vim are not shown
Solution: update gen_opt_test.vim and write test.log,
          let the makefile output test.log (Milly)

closes: #15887

Signed-off-by: Milly <milly.ca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/Make_mvc.mak b/src/testdir/Make_mvc.mak
index bb38e25..bbfd8f0 100644
--- a/src/testdir/Make_mvc.mak
+++ b/src/testdir/Make_mvc.mak
@@ -151,8 +151,9 @@
 	$(VIMPROG) -u gui_preinit.vim -U gui_init.vim $(NO_PLUGINS) -S runtest.vim $*.vim
 	@del vimcmd
 
-opt_test.vim: ../optiondefs.h gen_opt_test.vim
-	$(VIMPROG) -e -s -u NONE $(COMMON_ARGS) --nofork -S gen_opt_test.vim ../optiondefs.h
+opt_test.vim: gen_opt_test.vim ../optiondefs.h
+	$(VIMPROG) -e -s -u NONE $(COMMON_ARGS) --nofork -S $**
+	@if exist test.log ( type test.log & exit /b 1 )
 
 test_bench_regexp.res: test_bench_regexp.vim
 	-if exist benchmark.out del benchmark.out