test_runner: add Subtest to tap protocol output

lib/internal/test_runner/tap_stream.js

@@ -71,6 +71,10 @@ class TapStream extends Readable {
     return `TODO${reason ? ` ${tapEscape(reason)}` : ''}`;
   }
 
+  subtest(indent, name) {
+    this.#tryPush(`${indent}# Subtest: ${tapEscape(name)}\n`);
+  }
+
   details(indent, duration, error) {
     let details = `${indent}  ---\n`;
 

lib/internal/test_runner/test.js

@@ -183,6 +183,10 @@ class Test extends AsyncResource {
         return;
       }
 
+      if (i === 1 && this.parent !== null) {
+        this.reporter.subtest(this.indent, this.name);
+      }
+
       // Report the subtest's results and remove it from the ready map.
       subtest.finalize();
       this.readySubtests.delete(i);
@@ -416,6 +420,8 @@ class Test extends AsyncResource {
     // Output this test's results and update the parent's waiting counter.
     if (this.subtests.length > 0) {
       this.reporter.plan(this.subtests[0].indent, this.subtests.length);
+    } else {
+      this.reporter.subtest(this.indent, this.name);
     }
 
     this.report();

test/message/test_runner_no_refs.out

@@ -1,4 +1,6 @@
 TAP version 13
+# Subtest: does not keep event loop alive
+    # Subtest: +does not keep event loop alive
     not ok 1 - +does not keep event loop alive
       ---
       duration_ms: *

test/message/test_runner_only_tests.out

@@ -1,76 +1,97 @@
 TAP version 13
+# Subtest: only = undefined
 ok 1 - only = undefined # SKIP 'only' option not set
   ---
   duration_ms: *
   ...
+# Subtest: only = undefined, skip = string
 ok 2 - only = undefined, skip = string # SKIP 'only' option not set
   ---
   duration_ms: *
   ...
+# Subtest: only = undefined, skip = true
 ok 3 - only = undefined, skip = true # SKIP 'only' option not set
   ---
   duration_ms: *
   ...
+# Subtest: only = undefined, skip = false
 ok 4 - only = undefined, skip = false # SKIP 'only' option not set
   ---
   duration_ms: *
   ...
+# Subtest: only = false
 ok 5 - only = false # SKIP 'only' option not set
   ---
   duration_ms: *
   ...
+# Subtest: only = false, skip = string
 ok 6 - only = false, skip = string # SKIP 'only' option not set
   ---
   duration_ms: *
   ...
+# Subtest: only = false, skip = true
 ok 7 - only = false, skip = true # SKIP 'only' option not set
   ---
   duration_ms: *
   ...
+# Subtest: only = false, skip = false
 ok 8 - only = false, skip = false # SKIP 'only' option not set
   ---
   duration_ms: *
   ...
+# Subtest: only = true, skip = string
 ok 9 - only = true, skip = string # SKIP skip message
   ---
   duration_ms: *
   ...
+# Subtest: only = true, skip = true
 ok 10 - only = true, skip = true # SKIP
   ---
   duration_ms: *
   ...
+# Subtest: only = true, with subtests
+    # Subtest: running subtest 1
     ok 1 - running subtest 1
       ---
       duration_ms: *
       ...
+    # Subtest: running subtest 2
     ok 2 - running subtest 2
       ---
       duration_ms: *
       ...
+    # Subtest: skipped subtest 1
     ok 3 - skipped subtest 1 # SKIP 'only' option not set
       ---
       duration_ms: *
       ...
+    # Subtest: skipped subtest 2
     ok 4 - skipped subtest 2 # SKIP 'only' option not set
       ---
       duration_ms: *
       ...
+    # Subtest: running subtest 3
     ok 5 - running subtest 3
       ---
       duration_ms: *
       ...
+    # Subtest: running subtest 4
+        # Subtest: running sub-subtest 1
         ok 1 - running sub-subtest 1
           ---
           duration_ms: *
           ...
+        # Subtest: running sub-subtest 2
         ok 2 - running sub-subtest 2
           ---
           duration_ms: *
           ...
+        # Subtest: skipped sub-subtest 1
         ok 3 - skipped sub-subtest 1 # SKIP 'only' option not set
           ---
           duration_ms: *
           ...
+        # Subtest: skipped sub-subtest 2
         ok 4 - skipped sub-subtest 2 # SKIP 'only' option not set
           ---
           duration_ms: *
@@ -80,10 +101,12 @@ ok 10 - only = true, skip = true # SKIP
       ---
       duration_ms: *
       ...
+    # Subtest: skipped subtest 3
     ok 7 - skipped subtest 3 # SKIP 'only' option not set
       ---
       duration_ms: *
       ...
+    # Subtest: skipped subtest 4
     ok 8 - skipped subtest 4 # SKIP
       ---
       duration_ms: *