platformtest: add zfsGet bookmark handling & replicationCursor tests

This encodes the observation made in issue #230 :
In the ZFS version shipped in Ubuntu 16.04 where
`zfs get someprop a#bookmark` does not work.

platformtest/tests/replicationCursor.go

@@ -0,0 +1,62 @@
+package tests
+
+import (
+	"fmt"
+
+	"github.com/zrepl/zrepl/platformtest"
+	"github.com/zrepl/zrepl/zfs"
+)
+
+func ReplicationCursor(ctx *platformtest.Context) {
+
+	platformtest.Run(ctx, platformtest.PanicErr, ctx.RootDataset, `
+		CREATEROOT
+		+  "foo bar"
+		+  "foo bar@1 with space"
+	`)
+
+	ds, err := zfs.NewDatasetPath(ctx.RootDataset + "/foo bar")
+	if err != nil {
+		panic(err)
+	}
+	guid, err := zfs.ZFSSetReplicationCursor(ds, "1 with space")
+	if err != nil {
+		panic(err)
+	}
+	snapProps, err := zfs.ZFSGetCreateTXGAndGuid(ds.ToString() + "@1 with space")
+	if err != nil {
+		panic(err)
+	}
+	if guid != snapProps.Guid {
+		panic(fmt.Sprintf("guids to not match: %v != %v", guid, snapProps.Guid))
+	}
+
+	bm, err := zfs.ZFSGetReplicationCursor(ds)
+	if err != nil {
+		panic(err)
+	}
+	if bm.CreateTXG != snapProps.CreateTXG {
+		panic(fmt.Sprintf("createtxgs do not match: %v != %v", bm.CreateTXG, snapProps.CreateTXG))
+	}
+	if bm.Guid != snapProps.Guid {
+		panic(fmt.Sprintf("guids do not match: %v != %v", bm.Guid, snapProps.Guid))
+	}
+	if bm.Guid != guid {
+		panic(fmt.Sprintf("guids do not match: %v != %v", bm.Guid, guid))
+	}
+
+	// test nonexistent
+	err = zfs.ZFSDestroyFilesystemVersion(ds, bm)
+	if err != nil {
+		panic(err)
+	}
+	bm2, err := zfs.ZFSGetReplicationCursor(ds)
+	if bm2 != nil {
+		panic(fmt.Sprintf("expecting no replication cursor after deleting it, got %v", bm))
+	}
+	if err != nil {
+		panic(fmt.Sprintf("expecting no error for getting nonexistent replication cursor, bot %v", err))
+	}
+
+	// TODO test moving the replication cursor
+}