Make bootstrap system variables code cleaner

[morgo]

Enhancement

In #33769 two of the issues need to make changes to session/bootstrap.go so that they change the oom-action and run-auto-analyze settings for when bootstrapping in the test suite.

This made it very clear that this code is currently not very clean, and could be improved:

tidb/session/bootstrap.go

Lines 1967 to 2011 in d2ada35

	// Init global system variables table.
	values := make([]string, 0, len(variable.GetSysVars()))
	for k, v := range variable.GetSysVars() {
	// Only global variables should be inserted.
	if v.HasGlobalScope() {
	vVal := v.Value
	if v.Name == variable.TiDBTxnMode && config.GetGlobalConfig().Store == "tikv" {
	vVal = "pessimistic"
	}
	if v.Name == variable.TiDBRowFormatVersion {
	vVal = strconv.Itoa(variable.DefTiDBRowFormatV2)
	}
	if v.Name == variable.TiDBPartitionPruneMode {
	vVal = variable.DefTiDBPartitionPruneMode
	if flag.Lookup("test.v") != nil || flag.Lookup("check.v") != nil || config.CheckTableBeforeDrop {
	// enable Dynamic Prune by default in test case.
	vVal = string(variable.Dynamic)
	}
	}
	if v.Name == variable.TiDBEnableChangeMultiSchema {
	vVal = variable.Off
	if flag.Lookup("test.v") != nil || flag.Lookup("check.v") != nil {
	// enable change multi schema in test case for compatibility with old cases.
	vVal = variable.On
	}
	}
	if v.Name == variable.TiDBEnableAsyncCommit && config.GetGlobalConfig().Store == "tikv" {
	vVal = variable.On
	}
	if v.Name == variable.TiDBEnable1PC && config.GetGlobalConfig().Store == "tikv" {
	vVal = variable.On
	}
	if v.Name == variable.TiDBEnableMutationChecker {
	vVal = variable.On
	}
	if v.Name == variable.TiDBTxnAssertionLevel {
	vVal = variable.AssertionFastStr
	}
	value := fmt.Sprintf(`("%s", "%s")`, strings.ToLower(k), vVal)
	values = append(values, value)
	}
	}
	sql := fmt.Sprintf("INSERT HIGH_PRIORITY INTO %s.%s VALUES %s;", mysql.SystemDB, mysql.GlobalVariablesTable,
	strings.Join(values, ", "))
	mustExecute(s, sql)