Use square brackets in RUBY_DESCRIPTION

`EnvUtil::current_parser` uses a regexp to detect the presence of
`+PRISM`, but it is not expecting round brackets in the string.  We
workaround this by changing round brackets to square brackets.

We no longer need to exclude the test
`TestPatternMatching::test_alternative_pattern_nested` because it works
properly now with PRISM correctly detected.

Author: wks

test/.excludes-mmtk/TestPatternMatching.rb

@@ -34,7 +34,7 @@ def test_description
     ['--disable=mmtk', '--mmtk-max-heap=10240000']
   ]
 
-  MMTK_EXPECETED_PART = '+MMTk(XGC)'
+  MMTK_EXPECETED_PART = '+MMTk[XGC]'
 
   def test_enable
     ENABLE_OPTIONS.each do |version_args|
@@ -107,6 +107,6 @@ def test_gc_stat
   end
 
   def scrub_desc(desc)
-    desc.gsub(/\((MarkSweep|NoGC|Immix|StickyImmix)\)/, '(XGC)')
+    desc.gsub(/\[(MarkSweep|NoGC|Immix|StickyImmix)\]/, '[XGC]')
   end
 end

test/ruby/test_mmtk.rb

@@ -227,7 +227,7 @@ define_ruby_description(const char *const jit_opt)
         + rb_strlen_lit(" +MN")
 #if USE_MMTK
         // This should be long enough for all plans we have.
-        + rb_strlen_lit(" +MMTk(XXXXXXXXXXXXXXXX)")
+        + rb_strlen_lit(" +MMTk[XXXXXXXXXXXXXXXX]")
 #endif
         + rb_strlen_lit(" +PRISM")
 #if USE_MODULAR_GC
@@ -248,9 +248,9 @@ define_ruby_description(const char *const jit_opt)
     if (ruby_mn_threads_enabled) append(" +MN");
     if (rb_ruby_prism_p()) append(" +PRISM");
     WHEN_USING_MMTK({
-        append(" +MMTk(");
+        append(" +MMTk[");
         append(mmtk_plan_name());
-        append(")");
+        append("]");
     })
 #if USE_MODULAR_GC
     append(GC_DESCRIPTION);