tests/end_to_end: First test for the configuration

This first end-to-end test focuses in the functionality that is going to be
introduced in this branch: the use of a environment variable that will
overwrite, "on-the-fly", the current `settings.conf` file in the project's
root directory.
tests/run_test_suite.sh: Add information on test failures

The test added in the previous commit is faiing at the moment because I
still haven't implemented the necessary modifications to make it pass.
However, I've found it's quite hard to see what's failing, so this commit
adds some verbosity for the test failures.
settings.get_config: Read the settings from $MEGUP_CONFIG_FILE first

If this environment variable is set, `get_config()` must use it instead of
the config file in the project's root directory.
README.md: Update readme file with the latest changes

The section regarding configuration has been updated with the latest
changes. No one should need to dig into the code just know this
functionality exists.
settings.conf: Go back to using a single configuration file

Now, with all the changes in the previous commits, there's no need to keep
these two separate files.