tool/test/runner.rb had been copied from test/runner.rb.
test/runner.rb was for `make test-all`, and tool/test/runner.rb was for
`make test-testframework` and `make test-tool`.
But I want to avoid the code clones.
This change makes tool/test/runner.rb support --test-target-dir option
which allows tool/test/runner.rb to run `make test-all`.
Now we can remove test/runner.rb.
for test-all rule, we can specify a file with TESTS option like
`TESTS=test_xxx.rb`. However, we can eliminate last '.rb' suffix
so this patch try with '.rb' suffix if the given path is not available.
