schemachanger: Cleanup redundant tests and refactor it

This commit recognizes that there were previously three redundant
tests about concurrent schema changer behavior so it deletes them and
rewrite it to a simpler one.

Release note: None

Author: Xiang-Gu

pkg/sql/schemachanger/BUILD.bazel

@@ -39,39 +39,34 @@ go_test(
         "//pkg/ccl",
         "//pkg/jobs",
         "//pkg/jobs/jobspb",
+        "//pkg/keys",
+        "//pkg/kv",
         "//pkg/roachpb",
         "//pkg/security/securityassets",
         "//pkg/security/securitytest",
         "//pkg/server",
         "//pkg/sql",
         "//pkg/sql/catalog",
-        "//pkg/sql/catalog/descpb",
         "//pkg/sql/catalog/desctestutils",
         "//pkg/sql/execinfra",
-        "//pkg/sql/pgwire/pgcode",
         "//pkg/sql/rowenc",
         "//pkg/sql/schemachanger/scexec",
         "//pkg/sql/schemachanger/scop",
         "//pkg/sql/schemachanger/scplan",
         "//pkg/sql/schemachanger/sctest",  # keep
-        "//pkg/sql/sqltestutils",
+        "//pkg/sql/sessiondatapb",
         "//pkg/testutils",
         "//pkg/testutils/serverutils",
         "//pkg/testutils/sqlutils",
-        "//pkg/util",
         "//pkg/util/ctxgroup",
         "//pkg/util/leaktest",
         "//pkg/util/log",
         "//pkg/util/protoutil",
         "//pkg/util/randutil",
-        "//pkg/util/syncutil",
-        "@com_github_cockroachdb_cockroach_go_v2//crdb",
         "@com_github_cockroachdb_errors//:errors",
         "@com_github_cockroachdb_errors//errorspb",
-        "@com_github_lib_pq//:pq",
         "@com_github_stretchr_testify//assert",
         "@com_github_stretchr_testify//require",
-        "@org_golang_x_sync//errgroup",
     ],
 )