feat(logging): include test-id in log messages

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

1 files changed, 7 insertions, 2 deletions

diff --git a/src/nvim/os/pty_process_unix.c b/src/nvim/os/pty_process_unix.c
index 4a49c0b162..c5d6af0ff6 100644
--- a/src/nvim/os/pty_process_unix.c
+++ b/src/nvim/os/pty_process_unix.c
@@ -163,10 +163,15 @@ static struct termios termios_default;
 /// @param tty_fd   TTY file descriptor, or -1 if not in a terminal.
 void pty_process_save_termios(int tty_fd)
 {
-  DLOG("tty_fd=%d", tty_fd);
-  if (tty_fd == -1 || tcgetattr(tty_fd, &termios_default) != 0) {
+  if (tty_fd == -1) {
     return;
   }
+  int rv = tcgetattr(tty_fd, &termios_default);
+  if (rv != 0) {
+    ELOG("tcgetattr failed (tty_fd=%d): %s", tty_fd, strerror(errno));
+  } else {
+    DLOG("tty_fd=%d", tty_fd);
+  }
 }
 
 /// @returns zero on success, or negative error code