cue/format: add a regression test for issue 1544

This shows the current buggy behavior, where comprehensions at the start
of a file gain a blank space.

For #1544.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I9751f0eae5408c2c9e11b95e83107441e3209cc4
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1196107
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cue/format/testdata/issue1544.txtar

@@ -0,0 +1,22 @@
+-- if.input --
+if true {
+	x: 5
+}
+-- if.golden --
+ if true {
+	x: 5
+}
+-- let.input --
+let x = 5
+y: x
+-- let.golden --
+ let x = 5
+y: x
+-- for.input --
+for v in ["a", "b"] {
+	(v): v
+}
+-- for.golden --
+ for v in ["a", "b"] {
+	(v): v
+}