patch 8.1.1136: decoding of mouse click escape sequence is not tested

Problem:    Decoding of mouse click escape sequence is not tested.
Solution:   Add a test for xterm and SGR using low-level input.  Make
            low-level input execution with feedkeys() work.

src/testdir/test_termcodes.vim

@@ -0,0 +1,47 @@
+" Tests for decoding escape sequences sent by the terminal.
+
+" This only works for Unix in a terminal
+if has('gui_running') || !has('unix')
+  finish
+endif
+
+func Test_xterm_mouse_click()
+  new
+  let save_mouse = &mouse
+  let save_term = &term
+  let save_ttymouse = &ttymouse
+  set mouse=a
+  set term=xterm
+  call setline(1, ['line 1', 'line 2', 'line 3 is a bit longer'])
+  redraw
+
+  " Xterm mouse click
+  set ttymouse=xterm
+  let button = 0x20  " left down
+  let row = 2 + 32
+  let col = 6 + 32
+  call feedkeys("\<Esc>[M" .. list2str([button, col, row]), 'Lx!')
+
+  let button = 0x23  " release
+  call feedkeys("\<Esc>[M" .. list2str([button, col, row]), 'Lx!')
+
+  call assert_equal([0, 2, 6, 0], getpos('.'))
+
+  " SGR mouse click
+  set ttymouse=sgr
+  let button = 0  " left down
+  let row = 3
+  let col = 9
+  call feedkeys(printf("\<Esc>[<%d;%d;%dM", button, col, row), 'Lx!')
+
+  let button = 3  " release
+  call feedkeys(printf("\<Esc>[<%d;%d;%dm", button, col, row), 'Lx!')
+
+  call assert_equal([0, 3, 9, 0], getpos('.'))
+
+  let &mouse = save_mouse
+  let &term = save_term
+  let &ttymouse = save_ttymouse
+  bwipe!
+endfunc
+