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>