Travis disabled IPv6:

[ RUN      ] serverstart(), serverstop() parses endpoints correctly: FAIL
...build/neovim/neovim/test/functional/eval/server_spec.lua:83: Expected objects to be the same.
Passed in:
(table) {
  [1] = '127.0.0.1:12345' }
Expected:
(table) {
  [1] = '127.0.0.1:12345'
 *[2] = '::1:12345' }

Change all tests to ensure a server was actually started before
expecting it to be returned from serverlist().

With the old behavior, if a GUI makes a blocking request that requires user
interaction (like nvim_input()), it would not get any screen updates.

The client, not nvim, should decide how to handle notifications during a
pending request. If an rplugin wants to avoid async calls while a sync call is
busy, it likely wants to avoid processing async calls while another async call
also is handled as well.

This may break the expectation of some existing rplugins. For compatibility,
remote/define.vim reimplements the old behavior. Clients can opt-out by
specifying `sync=urgent`.

- Legacy hosts should be updated to use `sync=urgent`. They could add a flag
  indicating which async methods are always safe to call and which must wait
  until the main loop returns.
- New hosts can expose the full asyncness, they don't need to offer both
  behaviors.

ref #6532
ref #1398 d83868fe9071af1b4866594eac12f7aa0fa71b53

- test all properties
- test failure modes

...in order to retrieve highlights.

Added test/functional/api/highlight_spec.lua
HL_NORMAL is not really a good name, since it's more like an empty attribute than the normal's one.
If one pays attention, syn_cterm_attr2entry is never called with attr=0 because it's always special cased before.
I suggest in subsequent PRs we remove the ATTR_OFF and just insert an EMPTY ATTR/RESET_ATTR/UNINITIALIZED for id 0.

The "mapping" tests added in 541dde36e330 were flawed:
- Unlike op-pending mode, RPCs are _blocked_ during map-pending. So
  a synchronous RPC like nvim_get_current_buf() waits until
  'timeoutlen', then the mapping is canceled.
- helpers.expect() also performs a blocking RPC, so again that must not
  intervene the two nvim_input() calls.

closes #6166