feat: write results in separate files

[nqb]

When you use Venom on one system to run several test suites one by one, venom.log and test_results.XX are overwritten by latest test suite.

Steps to reproduce

# cat a.yml 
name: a
version: 2
testcases:
- name: a
  steps:
  - type: exec
    script: echo 'a'

# cat A.yml 
name: A
version: 2
testcases:
- name: A
  steps:
  - type: exec
    script: echo 'A' 

Run test suite one by one:

venom run -v --output-dir=results a.yml
venom run -v --output-dir=results A.yml

Actual results

results/test_results.xml and results/venom.log contains results from A.yml

Expected results

results directory contains results for a.yml and A.yml

Workaround

Run test suite at same time using venom run a.yml A.yml

[nqb]

It could be interesting to have a directory per test suite inside output_dir

[yesnault]

venom should create different file. test_result_a.xml and test_result_A.xml, same with venom.log file

[nqb]

Agree with your solution @yesnault 👍