roachtest: import-cancellation: use a fixed seed; smaller import

Currently, due to the randomness present in the test, there may be
significant variance in test runs. In some situations, the nature of the
random import cancellations may result in the test timing out. This
makes the test flaky.

Use a fixed seed for the test's PRNG to make the test more
deterministic.

Additionally, reduce the size of the overall import by lowering the
number of files used to generate the `lineitem` table. This should also
help stabilize the runtime.

Fix cockroachdb#90510.

Release note: None.

pkg/cmd/roachtest/tests/import_cancellation.go

@@ -83,7 +83,8 @@ func runImportCancellation(ctx context.Context, t test.Test, c cluster.Cluster)
 		t.Fatal(err)
 	}
 
-	rng, seed := randutil.NewPseudoRand()
+	seed := int64(1666467482296309000)
+	rng := randutil.NewTestRandWithSeed(seed)
 
 	tablesToNumFiles := map[string]int{
 		"region":   1,
@@ -93,7 +94,7 @@ func runImportCancellation(ctx context.Context, t test.Test, c cluster.Cluster)
 		"partsupp": 8,
 		"customer": 8,
 		"orders":   8,
-		"lineitem": 8,
+		"lineitem": 2,
 	}
 	for tbl := range tablesToNumFiles {
 		fixtureURL := fmt.Sprintf("gs://cockroach-fixtures/tpch-csv/schema/%s.sql?AUTH=implicit", tbl)