tests: Update check.valgrind with always-malloc and add logging options

Based on notes in https://wiki.gnome.org/Valgrind we need to use
always-malloc for valgrind runs.
Add check.valgrindlog and check.valgrindxml which output valgrind logs into
an ignored local tmp. Output logs are named <head-sha>-$TEST_NAMES.log so we
can track commits and use diff tools on the logs.