patch 8.1.1925: 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_environ.vim b/src/testdir/test_environ.vim
index 21bb09a..ba61b17 100644
--- a/src/testdir/test_environ.vim
+++ b/src/testdir/test_environ.vim
@@ -11,7 +11,7 @@
 
 func Test_getenv()
   unlet! $TESTENV
-  call assert_equal(v:null, getenv('TESTENV'))
+  call assert_equal(v:null, 'TESTENV'->getenv())
   let $TESTENV = 'foo'
   call assert_equal('foo', getenv('TESTENV'))
 endfunc