perf-test: initial version of memory test infrastructure.

The idea is run some code under valgrind --leak-check=full and report
a summary, leaving the user to peruse the log file if they want.

We go to some lengths to preserve the log files from accidental
overwriting; the full corpus takes about 3 hours to run under valgrind
on my machine.

The naming of the log directories may be slightly controversial; in
the unlikely event of two runs in less than a second, the log will be
overwritten. A previous version with mktemp+timestamp was dismissed as
overkill; just mktemp alone does not sort nicely.

One new test is included, to check notmuch new for memory leaks.