Redraw the screen each 20 retries

This allows us to eyeball why something is failing

tests/lib/plugin/shared.vim

@@ -51,7 +51,10 @@ func s:WaitForCommon(expr, assert, timeout)
     let start = reltime()
   endif
 
+  let iters = 0
+
   while 1
+    let iters += 1
     let errors_before = len( v:errors )
     if type(a:expr) == v:t_func
       let success = a:expr()
@@ -65,6 +68,10 @@ func s:WaitForCommon(expr, assert, timeout)
       return slept
     endif
 
+    if iters % 20 == 0
+      redraw!
+    endif
+
     if slept >= a:timeout
       break
     endif