Check for unit test log severity override earlier (#21177)
### Description
<!-- Describe your changes. -->
Setting the log level after environment creation is too late in some
cases.
If the DML EP is enabled, it will create a composite sink with the
original logger using the creation time log severity, as well as
additional ETW sink. As it saves the current severity levels for each
sink inside the composite sink that prevents being able to get verbose
log output to stdout even if you set that at the session level.
I don't know enough about the setup that combines ETW with the original
sink to say whether we should also be updating the severity of
individual sinks in the combined sink, so this change is limited to
making the unit tests behave in the expected manner when the default log
severity is set in the background and not directly controlled.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Make it possible to get verbose output to stdout when the DML EP is
enabled.
