testsuite: fix fileref unit test failure on tmpfs

Problem: the fileref unit test fails on tmpfs in debian 11,
kernel 6.1.0-rpi7-rpi-2712.

Although tmpfs supports sparse files, in some cases the test's efforts
to create sparse test files are unsuccessful.

Skip tests when the test file must be sparse and is not.

Fixes #5642

src/common/libutil/test/fileref.c

@@ -109,6 +109,28 @@ bool test_sparse (void)
     return result;
 }
 
+bool is_sparse (const char *name)
+{
+    bool result = false;
+#ifdef SEEK_HOLE
+    int fd;
+    struct stat sb;
+    off_t offset;
+
+    fd = open (mkpath (name), O_RDONLY);
+    if (fd < 0)
+        return false;
+    if (fstat (fd, &sb) < 0 || !S_ISREG (sb.st_mode))
+        goto error;
+    offset = lseek (fd, 0, SEEK_HOLE);
+    if (offset < sb.st_size)
+        result = true;
+error:
+    close (fd);
+#endif /* SEEK_HOLE */
+    return result;
+}
+
 /* Create test file 'name' under test directory.
  * Each character in  'spec' represents one block filled with that character,
  * except for "-" which tries to create a hole (if supported by file system).
@@ -478,10 +500,11 @@ void test_vec (void)
         json_t *o;
         bool rc;
 
-        skip (strchr (testvec[i].spec, '-') && !have_sparse,
-              1, "test directory does not support sparse files");
-
         mkfile ("testfile", 4096, testvec[i].spec);
+        skip ((strchr (testvec[i].spec, '-')
+              && (!have_sparse || !is_sparse ("testfile"))),
+              1, "sparse %s test file could not be created", testvec[i].spec);
+
         o = xfileref_create_vec (mkpath ("testfile"),
                              testvec[i].hashtype,
                              testvec[i].chunksize);
@@ -493,9 +516,8 @@ void test_vec (void)
             testvec[i].exp_blobs,
             testvec[i].hashtype);
         json_decref (o);
-        rmfile ("testfile");
-
         end_skip;
+        rmfile ("testfile");
     }
 }