fix(runner): check child status code after valgrind

[not-matthias]

Prints the valgrind logs now:

[DEBUG::codspeed::run::runner::valgrind::measure] Valgrind exit code = None, Valgrind signal = Some(11)
[DEBUG::codspeed::run::runner::valgrind::measure] valgrind.log: ==303374== Using source line as position.
--303374-- warning: L3 cache found, using its data for the LL simulation.
--303374-- warning: specified LL cache: line_size 64  assoc 12  total_size 18,874,368
--303374-- warning: simulated LL cache: line_size 64  assoc 18  total_size 18,874,368

Error: failed to execute valgrind

Before it only printed this:

Error: cannot parse integer from empty string

[art049]