Add an unique placeholder

[alexislefebvre]

I'm trying to generate code coverage with fastest.

@liuggio suggested to use --log-junit /tmp/logfile{p}.xml, but when I run 26 tests, the value {p} goes from 1 to 4, it looks like the file is overwritten by each test that use the same {p} value.

Currently they are these placeholders:

• {} is the current test file.
• {p} is the current processor number.

Can we have another placeholder to ease code coverage logging? I suggest to add another placeholder with an unique number or name, like the ENV_TEST_INC_NUMBER or ENV_TEST_CHANNEL_READABLE variables.

[liuggio]

@alexislefebvre :+1: would you like to provide a PR?

[alexislefebvre]

@liuggio OK I'll give it a try. Is it better to provide the number with a name (like ENV_TEST_CHANNEL_READABLE) or only a number (like ENV_TEST_INC_NUMBER)?

[liuggio]

@alexislefebvre I'd go for ENV_TEST_INC_NUMBER is a pure number, but I think is better if you decide :)

[liuggio]

@alexislefebvre great!