Fix test failure on Darwin

Apparently our DBD::SQLite links against /usr/lib/libsqlite3.dylib,
which is an old version that doesn't respect foreign key constraints.
So manifests/cache.sqlite doesn't get updated properly when a manifest
disappears.  We should fix our DBD::SQLite, but in the meantime this
will fix the test.

http://hydra.nixos.org/build/3017959

tests/binary-cache.sh

@@ -1,6 +1,7 @@
 source common.sh
 
 clearStore
+clearManifests
 
 # Create the binary cache.
 cacheDir=$TEST_ROOT/binary-cache