unittests: Do not hang when error message is too long

test/README.md

@@ -99,3 +99,6 @@ get backtrace from).
 approximately 90% of the tests. Should be used when finding cores is too hard 
 for some reason. Normally (on OS X or when `NVIM_TEST_CORE_GLOB_DIRECTORY` is 
 defined and this variable is not) cores are checked for after each test.
+
+`NVIM_TEST_RUN_TESTTEST` (U) (1): allows running `test/unit/testtest_spec.lua` 
+used to check how testing infrastructure works.

test/unit/helpers.lua

@@ -554,8 +554,6 @@ local function gen_itp(it)
         end
       else
         sc.close(wr)
-        sc.wait(child_pid)
-        child_pid = nil
         local function check()
           local res = sc.read(rd, 2)
           eq(2, #res)
@@ -570,6 +568,8 @@ local function gen_itp(it)
           assert.just_fail(err)
         end
         local err, emsg = pcall(check)
+        sc.wait(child_pid)
+        child_pid = nil
         sc.close(rd)
         if not err then
           if allow_failure then

test/unit/testtest_spec.lua

@@ -0,0 +1,13 @@
+local helpers = require('test.unit.helpers')(after_each)
+local assert = require('luassert')
+local itp = helpers.gen_itp(it)
+
+-- All of the below tests must fail. Check how exactly they fail.
+if os.getenv('NVIM_TEST_RUN_TESTTEST') ~= '1' then
+  return
+end
+describe('test code', function()
+  itp('does not hang when working with lengthy errors', function()
+    assert.just_fail(('x'):rep(65536))
+  end)
+end)