patch 8.0.1771: in tests, when WaitFor() fails it doesn't say why

Problem:    In tests, when WaitFor() fails it doesn't say why. (James McCoy)
Solution:   Add WaitForAssert(), which produces an assert error when it fails.

src/testdir/shared.vim

@@ -115,38 +115,80 @@ endfunc
 
 " Wait for up to five seconds for "expr" to become true.  "expr" can be a
 " stringified expression to evaluate, or a funcref without arguments.
+" Using a lambda works best.  Example:
+"	call WaitFor({-> status == "ok"})
+"
 " A second argument can be used to specify a different timeout in msec.
 "
-" Return time slept in milliseconds.  With the +reltime feature this can be
-" more than the actual waiting time.  Without +reltime it can also be less.
+" When successful the time slept is returned.
+" When running into the timeout an exception is thrown, thus the function does
+" not return.
 func WaitFor(expr, ...)
   let timeout = get(a:000, 0, 5000)
+  let slept = s:WaitForCommon(a:expr, v:null, timeout)
+  if slept < 0
+    throw 'WaitFor() timed out after ' . timeout . ' msec'
+  endif
+  return slept
+endfunc
+
+" Wait for up to five seconds for "assert" to return zero.  "assert" must be a
+" (lambda) function containing one assert function.  Example:
+"	call WaitForAssert({-> assert_equal("dead", job_status(job)})
+"
+" A second argument can be used to specify a different timeout in msec.
+"
+" Return zero for success, one for failure (like the assert function).
+func WaitForAssert(assert, ...)
+  let timeout = get(a:000, 0, 5000)
+  if s:WaitForCommon(v:null, a:assert, timeout) < 0
+    return 1
+  endif
+  return 0
+endfunc
+
+" Common implementation of WaitFor() and WaitForAssert().
+" Either "expr" or "assert" is not v:null
+" Return the waiting time for success, -1 for failure.
+func s:WaitForCommon(expr, assert, timeout)
   " using reltime() is more accurate, but not always available
+  let slept = 0
   if has('reltime')
     let start = reltime()
-  else
-    let slept = 0
   endif
-  if type(a:expr) == v:t_func
-    let Test = a:expr
-  else
-    let Test = {-> eval(a:expr) }
-  endif
-  for i in range(timeout / 10)
-    if Test()
-      if has('reltime')
-	return float2nr(reltimefloat(reltime(start)) * 1000)
-      endif
+
+  while 1
+    if type(a:expr) == v:t_func
+      let success = a:expr()
+    elseif type(a:assert) == v:t_func
+      let success = a:assert() == 0
+    else
+      let success = eval(a:expr)
+    endif
+    if success
       return slept
     endif
-    if !has('reltime')
+
+    if slept >= a:timeout
+      break
+    endif
+    if type(a:assert) == v:t_func
+      " Remove the error added by the assert function.
+      call remove(v:errors, -1)
+    endif
+
+    sleep 10m
+    if has('reltime')
+      let slept = float2nr(reltimefloat(reltime(start)) * 1000)
+    else
       let slept += 10
     endif
-    sleep 10m
-  endfor
-  throw 'WaitFor() timed out after ' . timeout . ' msec'
+  endwhile
+
+  return -1  " timed out
 endfunc
 
+
 " Wait for up to a given milliseconds.
 " With the +timers feature this waits for key-input by getchar(), Resume()
 " feeds key-input and resumes process. Return time waited in milliseconds.