Write fewer bits to test the rowcache behavior

The failure mode in question was pretty predictable and tied to number of
snapshots, not to number of bits written, so we can probably use a lot fewer
bits and still get good results, but this is really slow under -race testing.

fragment_internal_test.go

@@ -111,6 +111,11 @@ func TestFragment_ClearBit(t *testing.T) {
 func TestFragment_RowcacheMap(t *testing.T) {
 	var done int64
 	f := mustOpenFragment("i", "f", viewStandard, 0, "")
+	// Under -race, this test turns out to take a fairly long time
+	// to run with larger OpN, because we write 50,000 bits to
+	// the bitmap, and everything is being race-detected, and we don't
+	// actually need that many to get the result we care about.
+	f.MaxOpN = 2000
 	defer f.Clean(t)
 
 	ch := make(chan struct{})