Test: Create unique log files for nodeos

[heifner]

When nodeos is quickly relaunched in the integration tests, it is possible that a nodeos log file is overwritten because it has the same launch time and file name. This happens, for example, in nodeos_read_terminate_at_block_test.py. Make sure a unique log file name is created for each run of nodeos.

[ericpassmore]

Note:start
category: Tests
component: Internal
summary: Ensure unique nodeos log file names when running tests.
Note:end

[greg7mdp]

Maybe add a comment such as # make sure we don't overwrite an existing log file.