cue/format: add test case for formatting empty and near-empty files

They all behave as expected, but as I investigated the report at
https://cuelang.org/issue/3895 I noticed we lacked some test cases.

Note that the empty.input file has zero bytes, and empty.golden has one,
which is just a newline. Formatting empty CUE source files as one
newline seems reasonable given that we always end CUE source with
one newline. Modern text editors seem to do the same with empty files.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I58caeb2f2709a84adfd0f2aa223a8068e5de72bb
Dispatch-Trailer: {"type":"trybot","CL":1214303,"patchset":1,"ref":"refs/changes/03/1214303/1","targetBranch":"master"}

Author: mvdan

cue/format/testdata/comments.txtar

@@ -1,5 +1,8 @@
 simplify
 
+-- empty.input --
+-- empty.golden --
+
 -- comments.input --
 // Package line 1 group 1
 // Package line 2 group 1

cue/format/testdata/imports.txtar

@@ -1,5 +1,17 @@
 sort-imports
 
+-- pkgonly.input --
+package foo
+-- pkgonly.golden --
+package foo
+-- attronly.input --
+@attr(foo)
+-- attronly.golden --
+@attr(foo)
+-- importonly.input --
+import "foo"
+-- importonly.golden --
+import "foo"
 -- imports.input --
 package foo