Make mysql test optional again

[phbnf]

Unfortunately, the test is still flaky.

Undoes #821.

[roger2hk]

This PR doesn't actually make the MySQL test optional. The test will still run when there is a MySQL database available.

tessera/storage/mysql/mysql_test.go

Lines 65 to 84 in cd1dadb

	db, err := sql.Open("mysql", *mysqlURI)
	if err != nil {
	if *isMySQLTestOptional {
	klog.Warning("MySQL not available, skipping all MySQL storage tests")
	return
	}
	klog.Fatalf("Failed to open MySQL test db: %v", err)
	}
	defer func() {
	if err := db.Close(); err != nil {
	klog.Warningf("Failed to close MySQL database: %v", err)
	}
	}()
	if err := db.PingContext(ctx); err != nil {
	if *isMySQLTestOptional {
	klog.Warning("MySQL not available, skipping all MySQL storage tests")
	return
	}
	klog.Fatalf("Failed to ping MySQL test db: %v", err)
	}