test2json: use indentation to determine source of test output

If a test prints output after a subtest has started it was
being attributed to the last subtest. This change uses the identation
level of the output to determine which test the output comes from.

src/cmd/internal/test2json/test2json.go

@@ -211,8 +211,18 @@ func (c *converter) handleInputLine(line []byte) {
 		}
 	}
 
+	// Not a special test output line.
 	if !ok {
-		// Not a special test output line.
+		// Lookup the name of the test which produced the output using the
+		// indentation of the output as an index into the stack of the current
+		// subtests.
+		// If the indentation is greater than the number of current subtests
+		// then the output must have included extra indentation. We can't
+		// determine which subtest produced this output, so we default to the
+		// old behaviour of assuming the most recently run subtest produced it.
+		if indent > 0 && indent <= len(c.report) {
+			c.testName = c.report[indent-1].Test
+		}
 		c.output.write(origLine)
 		return
 	}

src/cmd/internal/test2json/testdata/issue29755.json

@@ -0,0 +1,15 @@
+{"Action":"run","Test":"TestOutputWithSubtest"}
+{"Action":"output","Test":"TestOutputWithSubtest","Output":"=== RUN   TestOutputWithSubtest\n"}
+{"Action":"run","Test":"TestOutputWithSubtest/sub_test"}
+{"Action":"output","Test":"TestOutputWithSubtest/sub_test","Output":"=== RUN   TestOutputWithSubtest/sub_test\n"}
+{"Action":"output","Test":"TestOutputWithSubtest","Output":"--- FAIL: TestOutputWithSubtest (0.00s)\n"}
+{"Action":"output","Test":"TestOutputWithSubtest","Output":"    foo_test.go:6: output before sub tests\n"}
+{"Action":"output","Test":"TestOutputWithSubtest/sub_test","Output":"    --- PASS: TestOutputWithSubtest/sub_test (0.00s)\n"}
+{"Action":"output","Test":"TestOutputWithSubtest/sub_test","Output":"        foo_test.go:9: output from sub test\n"}
+{"Action":"output","Test":"TestOutputWithSubtest","Output":"    foo_test.go:12: output after sub test\n"}
+{"Action":"pass","Test":"TestOutputWithSubtest/sub_test"}
+{"Action":"fail","Test":"TestOutputWithSubtest"}
+{"Action":"output","Output":"FAIL\n"}
+{"Action":"output","Output":"FAIL    gotest.tools/gotestsum/foo      0.001s\n"}
+{"Action":"output","Output":"FAIL\n"}
+{"Action":"fail"}

src/cmd/internal/test2json/testdata/issue29755.test

@@ -0,0 +1,10 @@
+=== RUN   TestOutputWithSubtest
+=== RUN   TestOutputWithSubtest/sub_test
+--- FAIL: TestOutputWithSubtest (0.00s)
+    foo_test.go:6: output before sub tests
+    --- PASS: TestOutputWithSubtest/sub_test (0.00s)
+        foo_test.go:9: output from sub test
+    foo_test.go:12: output after sub test
+FAIL
+FAIL    gotest.tools/gotestsum/foo      0.001s
+FAIL