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, 13 insertions, 13 deletions

diff --git a/test/functional/vimscript/let_spec.lua b/test/functional/vimscript/let_spec.lua
index 4ff4090a18..6e93655e32 100644
--- a/test/functional/vimscript/let_spec.lua
+++ b/test/functional/vimscript/let_spec.lua
@@ -47,33 +47,33 @@ describe(':let', function()
   end)
 
   it("multibyte env var #8398 #9267", function()
-    command("let $NVIM_TEST = 'AìaB'")
-    eq('AìaB', eval('$NVIM_TEST'))
-    command("let $NVIM_TEST = 'AaあB'")
-    eq('AaあB', eval('$NVIM_TEST'))
+    command("let $NVIM_TEST_LET = 'AìaB'")
+    eq('AìaB', eval('$NVIM_TEST_LET'))
+    command("let $NVIM_TEST_LET = 'AaあB'")
+    eq('AaあB', eval('$NVIM_TEST_LET'))
     local mbyte = [[\p* .ม .ม .ม .ม่ .ม่ .ม่ ֹ ֹ ֹ .ֹ .ֹ .ֹ ֹֻ ֹֻ ֹֻ
                     .ֹֻ .ֹֻ .ֹֻ ֹֻ ֹֻ ֹֻ .ֹֻ .ֹֻ .ֹֻ ֹ ֹ ֹ .ֹ .ֹ .ֹ ֹ ֹ ֹ .ֹ .ֹ .ֹ ֹֻ ֹֻ
                     .ֹֻ .ֹֻ .ֹֻ a a a ca ca ca à à à]]
-    command("let $NVIM_TEST = '"..mbyte.."'")
-    eq(mbyte, eval('$NVIM_TEST'))
+    command("let $NVIM_TEST_LET = '"..mbyte.."'")
+    eq(mbyte, eval('$NVIM_TEST_LET'))
   end)
 
   it("multibyte env var to child process #8398 #9267",  function()
-    local cmd_get_child_env = "let g:env_from_child = system(['"..nvim_dir.."/printenv-test', 'NVIM_TEST'])"
-    command("let $NVIM_TEST = 'AìaB'")
+    local cmd_get_child_env = "let g:env_from_child = system(['"..nvim_dir.."/printenv-test', 'NVIM_TEST_LET'])"
+    command("let $NVIM_TEST_LET = 'AìaB'")
     command(cmd_get_child_env)
-    eq(eval('$NVIM_TEST'), eval('g:env_from_child'))
+    eq(eval('$NVIM_TEST_LET'), eval('g:env_from_child'))
 
-    command("let $NVIM_TEST = 'AaあB'")
+    command("let $NVIM_TEST_LET = 'AaあB'")
     command(cmd_get_child_env)
-    eq(eval('$NVIM_TEST'), eval('g:env_from_child'))
+    eq(eval('$NVIM_TEST_LET'), eval('g:env_from_child'))
 
     local mbyte = [[\p* .ม .ม .ม .ม่ .ม่ .ม่ ֹ ֹ ֹ .ֹ .ֹ .ֹ ֹֻ ֹֻ ֹֻ
                     .ֹֻ .ֹֻ .ֹֻ ֹֻ ֹֻ ֹֻ .ֹֻ .ֹֻ .ֹֻ ֹ ֹ ֹ .ֹ .ֹ .ֹ ֹ ֹ ֹ .ֹ .ֹ .ֹ ֹֻ ֹֻ
                     .ֹֻ .ֹֻ .ֹֻ a a a ca ca ca à à à]]
-    command("let $NVIM_TEST = '"..mbyte.."'")
+    command("let $NVIM_TEST_LET = '"..mbyte.."'")
     command(cmd_get_child_env)
-    eq(eval('$NVIM_TEST'), eval('g:env_from_child'))
+    eq(eval('$NVIM_TEST_LET'), eval('g:env_from_child'))
   end)
 
   it("release of list assigned to l: variable does not trigger assertion #12387, #12430", function()