testing: make indentation consistent in sub-tests

Instead of mixed usage of spaces and tabs for indentation,
just use 4 spaces instead of one tab.

This test:

func TestX(t *testing.T) {
	t.Error("1\nnew line")
	t.Error("2")
	t.Error("3")
	t.Run("Y", func(t *testing.T) {
		t.Error("2")
		t.Error("2b\nnew line")
		t.Run("Z", func(t *testing.T) {
			t.Error("3\nnew line")
		})
	})
	t.Error("4")
}

produces following output:

--- FAIL: TestX (0.00s)
    indent_test.go:6: 1
	new line
    indent_test.go:7: 2
    indent_test.go:8: 3
    --- FAIL: TestX/Y (0.00s)
	indent_test.go:10: 2
	indent_test.go:11: 2b
	    new line
	--- FAIL: TestX/Y/Z (0.00s)
	    indent_test.go:13: 3
		new line
    indent_test.go:16: 4
FAIL

Fixes #25369

Change-Id: Ib3b5da45ab3ee670c6e8a23172e7cbefb94c5e60
Reviewed-on: https://go-review.googlesource.com/113177
Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

src/testing/testing.go

@@ -394,7 +394,7 @@ func (c *common) frameSkip(skip int) runtime.Frame {
 }
 
 // decorate prefixes the string with the file and line of the call site
-// and inserts the final newline if needed and indentation tabs for formatting.
+// and inserts the final newline if needed and indentation spaces for formatting.
 // This function must be called with c.mu held.
 func (c *common) decorate(s string) string {
 	frame := c.frameSkip(3) // decorate + log + public function.
@@ -414,8 +414,8 @@ func (c *common) decorate(s string) string {
 		line = 1
 	}
 	buf := new(strings.Builder)
-	// Every line is indented at least one tab.
-	buf.WriteByte('\t')
+	// Every line is indented at least 4 spaces.
+	buf.WriteString("    ")
 	fmt.Fprintf(buf, "%s:%d: ", file, line)
 	lines := strings.Split(s, "\n")
 	if l := len(lines); l > 1 && lines[l-1] == "" {
@@ -423,8 +423,8 @@ func (c *common) decorate(s string) string {
 	}
 	for i, line := range lines {
 		if i > 0 {
-			// Second and subsequent lines are indented an extra tab.
-			buf.WriteString("\n\t\t")
+			// Second and subsequent lines are indented an additional 4 spaces.
+			buf.WriteString("\n        ")
 		}
 		buf.WriteString(line)
 	}