[cairo] Cleaning up memleaks in test suite

[Andrea Canciani]

I started working on the patches for updating cairo-quartz-font to
work on Yosemite, but I wanted to ensure that the changes did not
introduce new problems.

While testing, I found out that if cairo-test-suite is run in
foreground (./cairo-test-suite -f), api-special-cases fails with the
assertion:

Assertion failed: (hash_table->live_entries == 0), function
_cairo_hash_table_destroy, file cairo-hash.c, line 217.
Abort trap: 6

I hunted some leaks down with valgrind (see following patches), but I
was unable to find the cause of this assertion, which seems to occour 
both on Linux and MacOSX.

I'd love to get rid of as many memleaks as possible before landing the
changes in cairo-quartz-font, as I'm afraid that I might miss some
reference leaks in the new code.

Moreover it's been quite some time since I worked on cairo, so I'd
love if somebody could review the patches.

Andrea