sql: re-enable splits in tests

Before:
```
make test PKG=./sql
go test -v  -i ./sql
go test  -run "." -timeout 1m10s ./sql
ok  	github.com/cockroachdb/cockroach/sql	24.985s
```
After:
```
make test PKG=./sql
go test -v  -i ./sql
go test  -run "." -timeout 1m10s ./sql
ok  	github.com/cockroachdb/cockroach/sql	30.452s
```

Closes #2504.

Makefile

@@ -28,7 +28,7 @@ STATIC :=
 PKG          := ./...
 TAGS         :=
 TESTS        := .
-TESTTIMEOUT  := 1m10s
+TESTTIMEOUT  := 2m
 RACETIMEOUT  := 5m
 BENCHTIMEOUT := 5m
 TESTFLAGS    :=

sql/config_test.go

@@ -83,9 +83,6 @@ func waitForConfigChange(t *testing.T, s *testServer) config.SystemConfig {
 // TestGetZoneConfig exercises config.GetZoneConfig and the sql hook for it.
 func TestGetZoneConfig(t *testing.T) {
 	defer leaktest.AfterTest(t)()
-	// Disable splitting. We're using bad attributes in zone configs
-	// to be able to match.
-	defer config.TestingDisableTableSplits()()
 	s, sqlDB, _ := setup(t)
 	defer cleanup(s, sqlDB)
 

sql/logic_test.go

@@ -38,7 +38,6 @@ import (
 	"time"
 	"unicode/utf8"
 
-	"github.com/cockroachdb/cockroach/config"
 	"github.com/cockroachdb/cockroach/security"
 	"github.com/cockroachdb/cockroach/server"
 	"github.com/cockroachdb/cockroach/sql"
@@ -1045,10 +1044,6 @@ func (t *logicTest) traceStop() {
 func TestLogic(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 
-	// TODO(marc): splitting ranges at table boundaries causes
-	// a blocked task and won't drain. Investigate and fix.
-	defer config.TestingDisableTableSplits()()
-
 	var globs []string
 	if *bigtest {
 		const logicTestPath = "../../sqllogictest"

sql/rename_test.go

@@ -35,7 +35,6 @@ import (
 // a rename operation.
 func TestRenameTable(t *testing.T) {
 	defer leaktest.AfterTest(t)()
-	defer config.TestingDisableTableSplits()()
 	_, sqlDB, kvDB, cleanup := sqlutils.SetupServer(t)
 	defer cleanup()
 

sql/trace_test.go

@@ -25,7 +25,6 @@ import (
 	"testing"
 	"text/tabwriter"
 
-	"github.com/cockroachdb/cockroach/config"
 	"github.com/cockroachdb/cockroach/util/leaktest"
 )
 
@@ -83,8 +82,6 @@ func prettyPrint(m [][]string) string {
 
 func TestExplainTrace(t *testing.T) {
 	defer leaktest.AfterTest(t)()
-	defer config.TestingDisableTableSplits()()
-
 	s, sqlDB, _ := setup(t)
 	defer cleanup(s, sqlDB)