Add ability to create a string transport logger for testing

[tripodsan]

Is your feature request related to a problem? Please describe.
for testing log output of the cli commands, it would we nice to have a string-only transport, so the test log files don't need to be read. this also would work around the flushing issue in winstonjs/winston#1504.

Describe the solution you'd like
when specifying /dev/text as logfile in Logger.getLogger(), a string based transport is created.
there should also be a way to retrieve the output, maybe by overriding the close() method so that:
const output = await log.close() returns the output.

Additional context

• see adobe/helix-cli@ce67700
• see intermittent failing logger test on circleci helix-cli#433