testers.testEqualContents: add checkMetadata option

Allows ignoring file metadata differences (permissions, ownership)
when comparing files.

This is especially useful on darwin, where we often run into subtle
issues like:

    -Device: 1,23 Access: (0444/-r--r--r--) Uid: ( 0/ root) Gid: ( 0/ wheel)
    +Device: 1,23 Access: (0444/-r--r--r--) Uid: ( 0/ root) Gid: ( 350/ nixbld)

doc/build-helpers/testers.chapter.md

@@ -421,6 +421,11 @@ Check that two paths have the same contents.
 
 : A message that is printed last if the file system object contents at the two paths don't match exactly.
 
+`checkMetadata` (boolean)
+
+: Whether to fail on metadata differences, such as permissions or ownership.
+  Defaults to `true`.
+
 :::{.example #ex-testEqualContents-toyexample}
 
 # Check that two paths have the same contents

pkgs/build-support/testers/default.nix

@@ -57,6 +57,7 @@
       actual,
       expected,
       postFailureMessage ? null,
+      checkMetadata ? true,
     }:
     runCommand "equal-contents-${lib.strings.toLower assertion}"
       {
@@ -66,12 +67,13 @@
           expected
           postFailureMessage
           ;
+        excludeMetadata = if checkMetadata then "no" else "yes";
         nativeBuildInputs = [ diffoscopeMinimal ];
       }
       ''
         echo "Checking:"
         printf '%s\n' "$assertion"
-        if ! diffoscope --no-progress --text-color=always --exclude-directory-metadata=no -- "$actual" "$expected"
+        if ! diffoscope --no-progress --text-color=always --exclude-directory-metadata="$excludeMetadata" -- "$actual" "$expected"
         then
           echo
           echo 'Contents must be equal, but were not!'