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, 5 insertions, 1 deletions

diff --git a/src/nvim/msgpack_rpc/server.c b/src/nvim/msgpack_rpc/server.c
index f15ce82917..3ced39117a 100644
--- a/src/nvim/msgpack_rpc/server.c
+++ b/src/nvim/msgpack_rpc/server.c
@@ -23,7 +23,6 @@
 
 #define MAX_CONNECTIONS 32
 #define ENV_LISTEN "NVIM_LISTEN_ADDRESS"  // deprecated
-#define ENV_NVIM "NVIM"
 
 static garray_T watchers = GA_EMPTY_INIT_VALUE;
 
@@ -56,6 +55,11 @@ bool server_init(const char *listen_addr)
     os_unsetenv(ENV_LISTEN);
   }
 
+  // TODO: this is for logging_spec. Can remove this after nvim_log #7062 is merged.
+  if (os_env_exists("__NVIM_TEST_LOG")) {
+    ELOG("test log message");
+  }
+
   return rv == 0;
 }