timezone: fix parallel check failures

The XT testdata install rules expect the testdata dir to already exist in
the build tree, but it doesn't actually create it.  Instead, it relies on
the build-testdata define happening to be executed before it (which runs
zic which creates the dir).  When we run in parallel though, it's easy to
hit a failure:
$ cd timezone
$ rm -rf $objdir/timezone/testdata
$ make check -j
...
cp testdata/XT1 .../timezone/testdata/XT1
cp: cannot create regular file '.../timezone/testdata/XT1': No such file or directory
Makefile:116: recipe for target '.../timezone/testdata/XT1' failed
make: *** [.../timezone/testdata/XT1] Error 1
make: *** Waiting for unfinished jobs....