Changed Vega formatting config for performance logging to be compatible with ReFrame 4.X

[casparvl]

See for an example of the new format: https://reframe-hpc.readthedocs.io/en/stable/configure.html

[smoors]

the alternative is to set 'perflog_compat': True to keep the old behavior, which i had to do in our local test suite for some tests that generate multiple performance values per run. having them all on 1 line breaks our elasticsearch monitoring setup.

but of course we don't know how long that will be supported..

[smoors]

lgtm, I'll just merge this and leave the actual testing in Vega to you :)