Make prove_installcheck remove the old log directory, if any.

prove_check already has been doing this.  Back-patch to 9.4, like the
commit that introduced this logging.

src/Makefile.global.in

@@ -351,6 +351,7 @@ endef
 ifeq ($(enable_tap_tests),yes)
 
 define prove_installcheck
+rm -rf $(CURDIR)/tmp_check/log
 cd $(srcdir) && TESTDIR='$(CURDIR)' PATH="$(bindir):$$PATH" PGPORT='6$(DEF_PGPORT)' top_builddir='$(CURDIR)/$(top_builddir)' PG_REGRESS='$(CURDIR)/$(top_builddir)/src/test/regress/pg_regress' $(PROVE) $(PG_PROVE_FLAGS) $(PROVE_FLAGS) t/*.pl
 endef