patch 8.0.1523: cannot write and read terminal screendumps

Problem:    Cannot write and read terminal screendumps.
Solution:   Add term_dumpwrite(), term_dumpread() and term_dumpdiff().
            Also add assert_equalfile().
diff --git a/src/proto/eval.pro b/src/proto/eval.pro
index ba7dcec..6f16ce0 100644
--- a/src/proto/eval.pro
+++ b/src/proto/eval.pro
@@ -122,6 +122,7 @@
 void prepare_assert_error(garray_T *gap);
 void assert_error(garray_T *gap);
 void assert_equal_common(typval_T *argvars, assert_type_T atype);
+void assert_equalfile(typval_T *argvars);
 void assert_match_common(typval_T *argvars, assert_type_T atype);
 void assert_inrange(typval_T *argvars);
 void assert_bool(typval_T *argvars, int isTrue);