When using --valgrind, you must also use --verbose iwd, and, depending
on the tests you may also need to include pytests in the verbose flag.
Since anyone using --valgrind definitely wants to see valgrind info
printed they should not need to enable verbose printing. Also, manually
parsing valgrind prints with IWD prints mixed throughout is a nightmare
even for a single test.
This patch uses valgrind's --log-file flag, which is directed to
/tmp/valgrind.log. After the tests runs we can print out this file.
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