Test colors were disabled in be7290d214673287218dcbd1a6730961cc067190
due to color codes interfering with the logs. I believe the solution to
disable colors is too aggressive. Reading raw logs isn't common compared
to reading CI results from the github UI. We should optimize the most
common use case.

It is also possible to interpret the colors codes in logs from neovim
with a plugin that interprets ansi escape sequences.

It previously gave a mix of forward and backslashes which was jarring.

Don't need to dumplog() on each failed test because we now have test-ids
that associate log messages with tests.

Problem:
1. Log messages (especially in CI) are hard to correlate with tests.
2. Since b353a5c05f02 #11886, dumplog() prints the logs next to test
   failures. This is noisy and gets in the way of the test results.

Solution:
1. Associate an incrementing id with each test and include it in log
   messages.
    - FUTURE: add v:name so Nvim instances can be formally "named"?
2. Mention "child" in log messages if the current Nvim is a child (based
   on the presence of $NVIM).

BEFORE:

    DBG … 12345      UI: event
    DBG … 12345      log_server_msg:722: RPC ->ch 1: …
    DBG … 12345      UI: flush
    DBG … 12345      inbuf_poll:444: blocking... events_enabled=1 events_pending=0
    DBG … 23454      UI: stop
    INF … 23454      os_exit:594: Nvim exit: 0

AFTER:

    DBG … T57        UI: event
    DBG … T57        log_server_msg:722: RPC ->ch 1: …
    DBG … T57        UI: flush
    DBG … T57        inbuf_poll:444: blocking... events_enabled=1 events_pending=0
    DBG … T57/child  UI: stop
    INF … T57/child  os_exit:594: Nvim exit: 0

Problem:
13748512f6d6 #15610 The no-colors codepath of the nvim.lua test output
handler does not handle nil, leading to weird symptoms if e.g. a test
has a syntax error:

    test/busted/outputHandlers/nvim.lua:105: attempt to concatenate a nil value

Solution:
Coerce to string in no-colors handler.

Problem
-------

Because test/busted/outputHandlers/nvim.lua doesn't know if it's running
in a terminal (no "isatty" equivalent), it outputs color codes in CI
logs and local tooling that runs the tests in a pipe:

    [1m[ SKIPPED ] 

This is just noise, hard for humans to read.

Solution
--------

Disable the color codes. If we later find a clever way to detect
a terminal in nvim.lua, we might consider re-enabling colors, but that
would still affect the CI build logs...