patch 8.1.2004: 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_expr.vim b/src/testdir/test_expr.vim
index 110cecb..365a813 100644
--- a/src/testdir/test_expr.vim
+++ b/src/testdir/test_expr.vim
@@ -53,7 +53,7 @@
 
 func Test_strgetchar()
   call assert_equal(char2nr('a'), strgetchar('axb', 0))
-  call assert_equal(char2nr('x'), strgetchar('axb', 1))
+  call assert_equal(char2nr('x'), 'axb'->strgetchar(1))
   call assert_equal(char2nr('b'), strgetchar('axb', 2))
 
   call assert_equal(-1, strgetchar('axb', -1))
@@ -63,7 +63,7 @@
 
 func Test_strcharpart()
   call assert_equal('a', strcharpart('axb', 0, 1))
-  call assert_equal('x', strcharpart('axb', 1, 1))
+  call assert_equal('x', 'axb'->strcharpart(1, 1))
   call assert_equal('b', strcharpart('axb', 2, 1))
   call assert_equal('xb', strcharpart('axb', 1))