manifestcache: clear the cache before testing the debug command

Right now the cache is empty before this section of the test. However, we are
about to improve the persistence of the cache (putting it under `wllock`,
intead of `lock`). So we install the cleanup before.

tests/test-manifest.t

@@ -108,6 +108,10 b' Showing the content of the caches after '
   $ hg debugmanifestfulltextcache
   cache empty
 
+(Clearing the cache in case of any content)
+
+  $ hg debugmanifestfulltextcache --clear
+
 Adding a new persistent entry in the cache
 
   $ hg debugmanifestfulltextcache --add 1e01206b1d2f72bd55f2a33fa8ccad74144825b7