Add support for test failure formatting

[FabienLydoire]

Hello,

The JUnit XML file format is not standardised across the heterogeneous testing tools.
In particular, the failure node is parsed in different ways by different tools although it's a critical component of the test report.

To solve this problem I propose to introduce formatting options.
Format:

• f : file_name
• l : line_number
• m : message

Usage:
trainer -assert_message_format "m (f:l)"
produces
<failure message="the assert message (file_name:12)"/>

trainer -assert_message_format "m" -failure_string_format "f:l"
produces
<failure message="the assert message">file_name:12</failure>

To cover even more situations, we could also add the failure type (But I don't think it's managed by the current version of the code).

What do you think ?