cue/format: add test for issue #4238

The formatter incorrectly places commas after inline comments in list
elements with ellipsis, e.g. `...string // comment` becomes
`...string // comment,` instead of `...string, // comment`.

For #4238

Signed-off-by: Daniel Martí <[e-mail]>
Change-Id: I6b1b887195fce3255d88090fc4eb930f446f2f69
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1229756
Reviewed-by: Matthew Sackman <[e-mail]>
TryBot-Result: CUEcueckoo <[e-mail]>
Unity-Result: CUE porcuepine <[e-mail]>

cue/format/testdata/comments.txtar

···
         [string]: _ // comment
     }
 }
+
+// issue #4238
+[string]: [
+	...string // inline comment without a comma
+]
+[string]: [
+	...string, // inline comment with a comma
+]
 -- out/format/comments.golden --
 // Package line 1 group 1
 // Package line 2 group 1
···
 
 	// an ellipsis
 }
+
+// issue #4238
+[string]: [
+	...string // inline comment without a comma,
+]
+[string]: [
+	...string // inline comment with a comma,
+]
 -- comment_alone.input --
 // Just one comment on its own.
 -- out/format/comment_alone.golden --