Disable test/xref2/labels.t under 4.07

Before OCaml 4.07, top-comments weren't recognized by the loader,
causing the following diff. This is potentially a bug.

    --- a/../../../default/test/xref2/labels.t/run.t
    +++ b/test/xref2/labels.t/run.t.corrected
    @@ -8,9 +8,8 @@ Some are not in order because the 'doc' field appears after the rest in the outp

       $ odoc_print test.odocl | jq -c '.. | .["`Heading"]? | select(.) | .[1]["`Label"]'
       [{"`Root":[{"`RootPage":"test"},"Test"]},"B"]
    -  [{"`Module":[{"`Root":[{"`RootPage":"test"},"Test"]},"M"]},"D"]
       [{"`Module":[{"`Root":[{"`RootPage":"test"},"Test"]},"M"]},"B"]
    -  [{"`Module":[{"`Root":[{"`RootPage":"test"},"Test"]},"M"]},"C"]
    +  [{"`Module":[{"`Root":[{"`RootPage":"test"},"Test"]},"M"]},"D"]
       [{"`Module":[{"`Root":[{"`RootPage":"test"},"Test"]},"N"]},"B"]
       [{"`Root":[{"`RootPage":"test"},"Test"]},"A"]

    @@ -23,9 +22,9 @@ We expect resolved references and the heading text filled in.
       [{"`Resolved":{"`Identifier":{"`Label":[{"`Root":[{"`RootPage":"test"},"Test"]},"A"]}}},[{"`Word":"First"},"`Space",{"`Word":"label"}]]
       [{"`Resolved":{"`Identifier":{"`Label":[{"`Root":[{"`RootPage":"test"},"Test"]},"B"]}}},[{"`Word":"Floating"},"`Space",{"`Word":"label"}]]
       [{"`Dot":[{"`Root":["M","`TUnknown"]},"C"]},[]]
    -  [{"`Resolved":{"`Label":[{"`Identifier":{"`Module":[{"`Root":[{"`RootPage":"test"},"Test"]},"M"]}},"D"]}},[]]
    +  [{"`Dot":[{"`Root":["M","`TUnknown"]},"D"]},[]]
       [{"`Resolved":{"`Label":[{"`Identifier":{"`Module":[{"`Root":[{"`RootPage":"test"},"Test"]},"M"]}},"B"]}},[]]
    -  [{"`Resolved":{"`Label":[{"`Identifier":{"`Module":[{"`Root":[{"`RootPage":"test"},"Test"]},"N"]}},"B"]}},[]]
    +  [{"`Dot":[{"`Root":["N","`TUnknown"]},"B"]},[]]

       $ odoc html-generate --indent -o html test.odocl

Author: Julow

test/xref2/dune

@@ -14,6 +14,12 @@
 (cram
  (deps %{bin:odoc} %{bin:odoc_print} %{bin:compile}))
 
+(subdir
+ v407_and_above
+ (cram
+  (enabled_if
+   (>= %{ocaml_version} 4.07.0))))
+
 (subdir
  v408_and_above
  (cram