Print an "in progress" message during reports creation process

[lepapareil]

Problem to solve

Actually --report-* options do not print any message when they are generating output files, so when a huge number of hurl files are executed, execution seems to be freezed between the end of the tests and the end of the output file generation (the hidden one).

This is even more noticeable when we run Hurl with the --test option, as there is a long delay between displaying the last success message and the test summary part.

Proposal

Print an "in progress" message like implicit parallel does when using --report-* and --test together.

[jcamiel]

I've done some experiment. With 1000 Hurl files to run, we're producing around 3000 files for the HTML report (1 timeline, 1 run, 1 source per test). With a flamegraph:

When we look at the report generation (the last second of the timeline), we can see that we're spending 64% of the time in the create file system call. It less us not a lot of room for optimisation, apart multi-threading the HTML report creation.

What we should do to improve things:

• at least: add a temporary message "Creating reports..."
• multithread the HTML report creation