cli: make the logging output of tests smarter.
cli: make the logging output of tests smarter.

This patch extends the changes from b0d39ca3201229635e42dd2d32152e72e8360dfb to the
tests in pkg/cli. CLI tests now use a `log.TestLogScope` which causes:

- the server logs to be output to files,
- the files to be automatically deleted when the test
  terminates successfully,
- the log directory to be reported in the error message when a test
  error occurs so that the user can go and look at the log files
  only if they want to.

vaibhavarora/cockroach,glycerine/cockroach,maxlang/cockroach,glycerine/cockroach,paperstreet/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,glycerine/cockroach,maxlang/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,glycerine/cockroach,paperstreet/cockroach,maxlang/cockroach,vaibhavarora/cockroach,maxlang/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,maxlang/cockroach,paperstreet/cockroach,maxlang/cockroach,glycerine/cockroach,paperstreet/cockroach,glycerine/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,glycerine/cockroach,glycerine/cockroach,maxlang/cockroach,maxlang/cockroach