sql: use seeded random for schemachange opsgen

A schemachange TestWorkload failure was difficult to
reproduce; to address this, this patch uses randutil's
`NewTestRand()` to allow for a global seed to be set when
stressing this test.

Epic: none

Release note: none

pkg/workload/schemachange/BUILD.bazel

@@ -30,6 +30,7 @@ go_library(
         "//pkg/sql/sem/tree",
         "//pkg/sql/types",
         "//pkg/util/encoding",
+        "//pkg/util/randutil",
         "//pkg/util/syncutil",
         "//pkg/util/timeutil",
         "//pkg/workload",

pkg/workload/schemachange/schemachange.go

@@ -18,7 +18,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
-	"math/rand"
 	"os"
 	"regexp"
 	"sync"
@@ -27,6 +26,7 @@ import (
 
 	"github.com/cockroachdb/cockroach/pkg/clusterversion"
 	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgcode"
+	"github.com/cockroachdb/cockroach/pkg/util/randutil"
 	"github.com/cockroachdb/cockroach/pkg/util/syncutil"
 	"github.com/cockroachdb/cockroach/pkg/util/timeutil"
 	"github.com/cockroachdb/cockroach/pkg/workload"
@@ -184,7 +184,10 @@ func (s *schemaChange) Ops(
 	if err != nil {
 		return workload.QueryLoad{}, err
 	}
-	ops := newDeck(rand.New(rand.NewSource(timeutil.Now().UnixNano())), opWeights...)
+	stdoutLog := makeAtomicLog(os.Stdout)
+	rng, seed := randutil.NewTestRand()
+	stdoutLog.printLn(fmt.Sprintf("using random seed: %d", seed))
+	ops := newDeck(rng, opWeights...)
 	// A separate deck is constructed of only schema changes supported
 	// by the declarative schema changer. This deck has equal weights,
 	// only for supported schema changes.
@@ -194,11 +197,10 @@ func (s *schemaChange) Ops(
 			declarativeOpWeights[idx] = weight
 		}
 	}
-	declarativeOps := newDeck(rand.New(rand.NewSource(timeutil.Now().UnixNano())), declarativeOpWeights...)
+	declarativeOps := newDeck(rng, declarativeOpWeights...)
 
 	ql := workload.QueryLoad{SQLDatabase: sqlDatabase}
 
-	stdoutLog := makeAtomicLog(os.Stdout)
 	var artifactsLog *atomicLog
 	if s.logFilePath != "" {
 		err := s.initJSONLogFile(s.logFilePath)
@@ -215,7 +217,7 @@ func (s *schemaChange) Ops(
 			seqNum:             seqNum,
 			errorRate:          s.errorRate,
 			enumPct:            s.enumPct,
-			rng:                rand.New(rand.NewSource(timeutil.Now().UnixNano())),
+			rng:                rng,
 			ops:                ops,
 			declarativeOps:     declarativeOps,
 			maxSourceTables:    s.maxSourceTables,