patch 8.1.1987: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
diff --git a/src/testdir/test_reltime.vim b/src/testdir/test_reltime.vim
index 404ea3f..e25c3ce 100644
--- a/src/testdir/test_reltime.vim
+++ b/src/testdir/test_reltime.vim
@@ -8,11 +8,11 @@
   let now = reltime()
   sleep 10m
   let later = reltime()
-  let elapsed = reltime(now)
+  let elapsed = now->reltime()
   call assert_true(reltimestr(elapsed) =~ '0\.0')
-  call assert_true(reltimestr(elapsed) != '0.0')
+  call assert_true(elapsed->reltimestr() != '0.0')
   call assert_true(reltimefloat(elapsed) < 0.1)
-  call assert_true(reltimefloat(elapsed) > 0.0)
+  call assert_true(elapsed->reltimefloat() > 0.0)
 
   let same = reltime(now, now)
   call assert_equal('0.000', split(reltimestr(same))[0][:4])