DB error when test mail at install process

at install dialog, input email info (user, password) and click test or save
test mail process issue message:
"DB.getDatabase: No Database Connection"
and test for sent email will always fail

reason: sent email read some configuration from database like MAIL_SEND_BCC_TO_FROM.
but database is not finish yet configuration

solution: add flag isTestEmail to Email class to distinguish case test email when install