* test/runtest: cat test-temp-dir only once. Destroy and create

test Mail hierarchy here, so each test has its own independent
test data.

* test/setup-test: Drop needless pushd/popd, which don't exist in
sh anyway.  Don't use aclocal, which is part of automake.
Don't create Mail hierarchy here (see test/runtest).

* test/tests/folder/test-create, tests/folder/test-total: Drop
unused BINDIR.

* tests/pick/test-pick: Remove failing no-op test.

* test/tests/scan/test-scan: Write expected and actual output to
files and diff them.  This way, if the output differs, diff
provides the FAILing exit code and shows the differences.