bisync: disable --sftp-copy-is-hardlink on sftp tests

Before this change, TestSFTPOpenssh integration tests would fail due to setting
copy_is_hardlink=true in /fstest/testserver/init.d/TestSFTPOpenssh.

For example, if a file was server-side copied from path1 to path2 and then the
bisync tests set the path2 modtime, the path1 modtime would also unexpectedly
mutate.

Hardlinks are not the same as copies. The bisync tests assume that they can
modify a file on one side without affecting a file on the other. This change
essentially sets --sftp-copy-is-hardlink to the default of false for the bisync
tests.

cmd/bisync/bisync_test.go

@@ -953,6 +953,12 @@ func (b *bisyncTest) checkPreReqs(ctx context.Context, opt *bisync.Options) (con
 		b.fs2.Features().Disable("Copy") // API has longstanding bug for conflictBehavior=replace https://github.com/rclone/rclone/issues/4590
 		b.fs2.Features().Disable("Move")
 	}
+	if strings.HasPrefix(b.fs1.String(), "sftp") {
+		b.fs1.Features().Disable("Copy") // disable --sftp-copy-is-hardlink as hardlinks are not truly copies
+	}
+	if strings.HasPrefix(b.fs2.String(), "sftp") {
+		b.fs2.Features().Disable("Copy") // disable --sftp-copy-is-hardlink as hardlinks are not truly copies
+	}
 	if strings.Contains(strings.ToLower(fs.ConfigString(b.fs1)), "mailru") || strings.Contains(strings.ToLower(fs.ConfigString(b.fs2)), "mailru") {
 		fs.GetConfig(ctx).TPSLimit = 10 // https://github.com/rclone/rclone/issues/7768#issuecomment-2060888980
 	}

fstest/test_all/config.yaml

@@ -322,7 +322,6 @@ backends:
    remote:   "TestSFTPOpenssh:"
    fastlist: false
    ignoretests:
-     - cmd/bisync
      - cmd/gitannex
  - backend:  "sftp"
    remote:   "TestSFTPRclone:"