以下のように、./ruby test/runner.rb -j 2 test/digest とすると、
テストのファイルを見つけられないようです。

% ./ruby test/runner.rb -j 2 test/digest
Run options: -j 2

# Running tests:


Retrying...


test/digest/test_digest.rb: cannot load such file --
test/digest/test_digest.rb (LoadError)
test/digest/test_digest_hmac.rb: cannot load such file --
test/digest/test_digest_hmac.rb (LoadError)
test/digest/test_digest_extend.rb: cannot load such file --
test/digest/test_digest_extend.rb (LoadError)



Finished tests in 0.986568s, 0.0000 tests/s, 0.0000 assertions/s.

0 tests, 0 assertions, 0 failures, 0 errors, 0 skips

ruby -v: ruby 2.0.0dev (2011-11-01 trunk 33603) [x86_64-linux]

以下のように、-j を指定しなければ問題なく動きます。

% ./ruby test/runner.rb test/digest
Run options:

# Running tests:

..................................................................................

Finished tests in 0.063393s, 1293.5130 tests/s, 4432.6482 assertions/s.

82 tests, 281 assertions, 0 failures, 0 errors, 0 skips

ruby -v: ruby 2.0.0dev (2011-11-01 trunk 33603) [x86_64-linux]
-- 
[田中 哲][たなか あきら][Tanaka Akira]