patch 8.0.0156: not enough test coverage for float functions

Problem:    Several float functions are not covered by tests.
Solution:   Add float tests. (Dominique Pelle)
diff --git a/src/Makefile b/src/Makefile
index 3f61de6..7ae3fcb 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2104,6 +2104,7 @@
 	test_fileformat \
 	test_filter_cmd \
 	test_filter_map \
+	test_float_func \
 	test_fnameescape \
 	test_fnamemodify \
 	test_fold \