tests: write logs to files

tests: write logs to files

This cleans up the 'make check' output considerably. When all goes well,
you will only see the "PASS" line for each of $TESTS.

Weston logs into a separate file than stdout and stderr, so server logs
end up in one file per test, and other output to another file per test.

'make distclean' does not remove the tests/logs/ directory.

Also changes the weston-tests-env interpreter to bash, since I think &>
and ${1/.la/.so} might be bashisms.