There is a test directory named test/-ext-. Because this directry
starts with a hyphen, we have to cheat test/unit in order for it to
run the tests underneath. TESTS='-- -ext' worked for a long time.
Let's not break that maneuver.
`make test-all TESTS=name` can specify running test files by name.
name can be dirname ('dir/') or a file ('.../test_foo.rb'). This
patch complement `test_` prefix for a test. So we only need to
specify `TESTS=ruby/hash` which means `TESTS=ruby/test_hash.rb`.
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.
