Added support for providing the test configuration file with --config parameter

[juhamust]

• Adds new parameter to fl-run-bench and fl-run-test executables: --config
• Not using short parameter option "-c" because of conflicting with --cycles/-c
• Updated documentation accordingly

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Reasoning for the change:

• If find it limiting use configuration files named after the class. One may want to have named them after setup
• I wasn't aware of FL_CONF_PATH environment variable until opened the source code. Providing a parameter is much easier to find and use

[bdelbosc]

Thanks for your contribution Juha.