unittests: Do not hang when error message is too long

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)