tests: Skip hole-related port tests on Darwin.

Hole are itself a file-system specific feature and they are not
mandated.  While APFS does support sparse files, they do not behave like
on Linux.  I did not discover exact rules, but the file needs to be
large (100s of kB at least) and the holes are not aligned as the test
code expects.  So just disable them.

* test-suite/tests/ports.test (skip-on-darwin): New procedure.
("size of sparse file", "SEEK_DATA while on data")
("SEEK_DATA while in hole", "SEEK_HOLE while in hole"): Skip on Darwin.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

test-suite/tests/ports.test

@@ -39,6 +39,10 @@
 (define (test-file)
   (data-file-name "ports-test.tmp"))
 
+(define (skip-on-darwin)
+  (when (string-ci=? "darwin" (utsname:sysname (uname)))
+    (throw 'untested)))
+
 
 ;;;; Some general utilities for testing ports.
 
@@ -185,6 +189,7 @@
     (close-port iport))
   (delete-file filename))
 
+;;; Note: Holes are weird on Darwin.
 (let* ((file (test-file))
        (port (open-output-file file)))
   (seek port 4096 SEEK_SET)
@@ -193,10 +198,15 @@
 
   (pass-if-equal "size of sparse file"
       4100
+    ;; XXX: On macOS, APFS does support sparse files, they do not behave
+    ;; like on Linux.  Skip these tests on macOS.
+    (skip-on-darwin)
+
     (stat:size (stat file)))
 
   (pass-if-equal "SEEK_DATA while on data"
       4096
+    (skip-on-darwin)
     (if (defined? 'SEEK_DATA)
         (call-with-input-file file
           (lambda (port)
@@ -209,6 +219,7 @@
 
   (pass-if-equal "SEEK_DATA while in hole"
       4096
+    (skip-on-darwin)
     (if (defined? 'SEEK_DATA)
         (call-with-input-file file
           (lambda (port)
@@ -221,6 +232,7 @@
 
   (pass-if-equal "SEEK_HOLE while in hole"
       10
+    (skip-on-darwin)
     (if (defined? 'SEEK_HOLE)
         (call-with-input-file file
           (lambda (port)