tests: Test `awful.placement.closest_corner`

The output image look fine, some `assert()` could be added.

tests/examples/awful/placement/closest_mouse.lua

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+local awful = {placement = require("awful.placement")} --DOC_HIDE
+
+local c = client.gen_fake {x = 20, y = 20, width= 280, height=200, screen =screen[1]} --DOC_HIDE
+local bw = c.border_width --DOC_HIDE
+
+-- Left  --DOC_HIDE
+mouse.coords {x=100,y=100} --DOC_HIDE
+ -- Move the mouse to the closest corner of the focused client
+awful.placement.closest_corner(mouse, {include_sides=true, parent=c})
+mouse.push_history() --DOC_HIDE
+assert(mouse.coords().x == c.x-bw and mouse.coords().y == (c.y) + (c.height)/2) --DOC_HIDE
+
+-- Top left --DOC_HIDE
+mouse.coords {x=60,y=40} --DOC_HIDE
+awful.placement.closest_corner(mouse, {include_sides=true, parent=c}) --DOC_HIDE
+mouse.push_history() --DOC_HIDE
+assert(mouse.coords().x == c.x-bw and mouse.coords().y == c.y-bw) --DOC_HIDE
+
+-- Top right --DOC_HIDE
+mouse.coords {x=230,y=50} --DOC_HIDE
+awful.placement.closest_corner(mouse, {include_sides=true, parent=c}) --DOC_HIDE
+mouse.push_history() --DOC_HIDE
+assert(mouse.coords().x == c.x+c.width+bw and mouse.coords().y == c.y-bw) --DOC_HIDE
+
+-- Right --DOC_HIDE
+mouse.coords {x=240,y=140} --DOC_HIDE
+awful.placement.closest_corner(mouse, {include_sides=true, parent=c}) --DOC_HIDE
+mouse.push_history() --DOC_HIDE
+assert(mouse.coords().x == c.x+c.width+bw and mouse.coords().y == c.y+c.height/2) --DOC_HIDE
+
+-- Bottom right --DOC_HIDE
+mouse.coords {x=210,y=190} --DOC_HIDE
+awful.placement.closest_corner(mouse, {include_sides=true, parent=c}) --DOC_HIDE
+mouse.push_history() --DOC_HIDE
+assert(mouse.coords().x == c.x+c.width+bw and mouse.coords().y == c.y+c.height+bw) --DOC_HIDE
+
+-- Bottom --DOC_HIDE
+mouse.coords {x=130,y=190} --DOC_HIDE
+awful.placement.closest_corner(mouse, {include_sides=true, parent=c}) --DOC_HIDE
+mouse.push_history() --DOC_HIDE
+assert(mouse.coords().x == c.x+c.width/2 and mouse.coords().y == c.y + c.height + bw) --DOC_HIDE
+
+-- Top --DOC_HIDE
+mouse.coords {x=130,y=30} --DOC_HIDE
+awful.placement.closest_corner(mouse, {include_sides=true, parent=c}) --DOC_HIDE
+mouse.push_history() --DOC_HIDE
+assert(mouse.coords().x == c.x + c.width/2 and mouse.coords().y == c.y-bw) --DOC_HIDE
+
+-- Bottom left + outside of "c" --DOC_HIDE
+mouse.coords {x=0,y=230} --DOC_HIDE
+awful.placement.closest_corner(mouse, {include_sides=true, parent=c}) --DOC_HIDE
+mouse.push_history() --DOC_HIDE
+assert(mouse.coords().x == c.x-bw and mouse.coords().y == c.y+c.height+bw) --DOC_HIDE
+ 
+ -- It is possible to emulate the mouse API to get the closest corner of
+ -- random area
+local corner = awful.placement.closest_corner(
+    {coords=function() return {x = 100, y=100} end},
+    {include_sides = true, bounding_rect = {x=0, y=0, width=200, height=200}}
+)
+print("Closest corner:", corner)