patch 8.1.1909: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make a few more functions usable as a method.
diff --git a/src/testdir/test_assert.vim b/src/testdir/test_assert.vim
index 65a0ee4..d89bd98 100644
--- a/src/testdir/test_assert.vim
+++ b/src/testdir/test_assert.vim
@@ -54,7 +54,7 @@
 
   let goodtext = ["one", "two", "three"]
   call writefile(goodtext, 'Xone')
-  call assert_equal(1, assert_equalfile('Xone', 'xyzxyz'))
+  call assert_equal(1, 'Xone'->assert_equalfile('xyzxyz'))
   call assert_match("E485: Can't read file xyzxyz", v:errors[0])
   call remove(v:errors, 0)
 
@@ -97,6 +97,9 @@
   call assert_equal(1, assert_report('something is wrong'))
   call assert_match('something is wrong', v:errors[0])
   call remove(v:errors, 0)
+  call assert_equal(1, 'also wrong'->assert_report())
+  call assert_match('also wrong', v:errors[0])
+  call remove(v:errors, 0)
 endfunc
 
 func Test_assert_exception()
diff --git a/src/testdir/test_bufline.vim b/src/testdir/test_bufline.vim
index a6b4694..6d62e51 100644
--- a/src/testdir/test_bufline.vim
+++ b/src/testdir/test_bufline.vim
@@ -152,11 +152,11 @@
 
   let lines =<< trim END
     new foo
-    let winnr=bufwinnr('foo')
-    let buf=bufnr('foo')
+    let winnr = 'foo'->bufwinnr()
+    let buf = bufnr('foo')
     wincmd p
     call appendbufline(buf, '$', range(1,200))
-    exe winnr. 'wincmd w'
+    exe winnr .. 'wincmd w'
     norm! G
     wincmd p
     call deletebufline(buf, 1, '$')
diff --git a/src/testdir/test_bufwintabinfo.vim b/src/testdir/test_bufwintabinfo.vim
index 0e8c7d1..7f0a6ce 100644
--- a/src/testdir/test_bufwintabinfo.vim
+++ b/src/testdir/test_bufwintabinfo.vim
@@ -18,7 +18,7 @@
     let l = getbufinfo('%')
     call assert_equal(bufnr('%'), l[0].bufnr)
     call assert_equal('vim', l[0].variables.editor)
-    call assert_notequal(-1, index(l[0].windows, bufwinid('%')))
+    call assert_notequal(-1, index(l[0].windows, '%'->bufwinid()))
 
     " Test for getbufinfo() with 'bufmodified'
     call assert_equal(0, len(getbufinfo({'bufmodified' : 1})))
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index f742035..23041d1 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -2332,8 +2332,7 @@
 endfunc
 
 func Test_popupwin_recycle_bnr()
-  let winid = popup_notification('nothing wrong', {})
-  let bufnr = winbufnr(winid)
+  let bufnr = popup_notification('nothing wrong', {})->winbufnr()
   call popup_clear()
   let winid = popup_notification('nothing wrong', {})
   call assert_equal(bufnr, winbufnr(winid))