Make racc test more flexible (for JRuby).

JRuby uses these same files for testing racc. The existing logic will not
find 'racc' in a JRuby project checkout. This change allows it to work by
just assuming 'ruby -S racc' when running tests. This will not change C
Ruby's detection when setting up tests (since earlier checks will find
racc).

Author: enebo

test/case.rb

@@ -17,6 +17,8 @@ class TestCase < Test::Unit::TestCase
     TEST_DIR = test_dir
     racc = File.join(PROJECT_DIR, 'bin', 'racc')
     racc = File.join(PROJECT_DIR, '..', 'libexec', 'racc') unless File.exist?(racc)
+    racc = 'racc' unless File.exist?(racc)
+
     RACC = racc
     ASSET_DIR = File.join(TEST_DIR, 'assets') # test grammars
     REGRESS_DIR  = File.join(TEST_DIR, 'regress') # known-good generated outputs