CtsTestServer: Add support for an alternate redirect route

Doing redirects from /redirect/assets/file to /assets/file by
stripping path segments is too symmetrical for some tests.
For instance, a relative reference "otherfile", relative to
/assets/file (loaded via /redirect/assets/file) might incorrectly
be loaded from /redirect/assets/otherfile which will work only
by coincidence. In general there's no such guarantee that such
redirects map any pathname symmetrically.

Change-Id: I9890d59ac0441ce9e6218c73deb590555af7f4a5