patch 8.0.0167: str2nr()/str2float() fail with negative values

Problem:    str2nr() and str2float() do not always work with negative values.
Solution:   Be more flexible about handling signs. (LemonBoy, closes #1332)
            Add more tests.
diff --git a/src/Makefile b/src/Makefile
index 7ae3fcb..db70672 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2091,6 +2091,7 @@
 	test_delete \
 	test_diffmode \
 	test_digraph \
+	test_functions \
 	test_display \
 	test_ex_undo \
 	test_execute_func \