Improve dataset key reporting in case of failure.

Currently, if a test using a dataset provider fail, atoum display in cli
the following message ($ is the CLI prompt):

$ In file 547c9209a296c on line 547 in case '547c9209a276c', 547c9209a29d7 failed for data set #3404808712286633985 of data provider 547c9209a27e1: 547c9209a2a00

To be clear, it display the key of the dataset which fail as # following by a string if a named key is used, by a number if the key is not named.
And if it's easily readable if key are not named, it's not the case if
the key is named:

$ In file 547c9209a296c on line 547 in case '547c9209a276c', 547c9209a29d7 failed for data set #integer should be greater than 0 of data provider 547c9209a27e1: 547c9209a2a00

So, to improve readability, this commit use the [] notation for the two kind of key:

$ In file 547c9209a296c on line 547 in case '547c9209a276c', 547c9209a29d7 failed for data set [integer should be greater than 0] of data provider 547c9209a27e1: 547c9209a2a00