tests: Look for httpcache in ${test_builddir}, not PATH

For installed-tests, the installed test directory is not on the PATH.
To make this easier, put the uninstalled binary in tests/, so that
in both build-time and installed tests, it is in ${test_builddir}.

Signed-off-by: Simon McVittie <smcv@debian.org>

Closes: #1989
Approved by: alexlarsson
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