Provide a way to fail a test via `template`

[GregoryAlbouy]

Description

See #82 for context.

Once a benchmark done, we display a default summary featuring some basic stats (min, max, mean durations).
This summary can be overridden by a custom template with access to all output.Report data.
However there is currently no way to actually fail the test. It has to be done by the user's script reading the output and deciding whether it should fail.
We want to address that by providing a way to fail the test depending on results.

Tasks

• Implement a template function fail that fails a test when called by the user in the template
• Make sure it results in os.Exit(1)

Suggestions

Notes

• Part of #82