read the feature flags in sextest in the way sexc does

sextest resolves #+ and #- itself -- it reads the program and prints
what survives to sexc -- so a flag spelling it does not recognise
decides which branch gets compiled

Makefile

@@ -90,7 +90,8 @@ sextest: $(EGGS_STAMP)
 	$(MAKE) -C ./tools/sextest sextest
 	cp ./tools/sextest/sextest .
 
-SEX_TEST_PROGRAMS = hello-world lists comments unicode serialize features
+SEX_TEST_PROGRAMS = hello-world lists comments unicode serialize features \
+                    feature-flags
 
 # Multi-module linking is checked end to end; see tests/modules/Makefile.
 check-modules: sexc