Make TracePoint#enable with block target current thread by default

If TracePoint#enable is passed a block, it previously started
the trace on all threads.  This changes it to trace only the
current thread by default.  To limit the scope of the change,
the current thread is only used by default if target and
target_line are both nil.  You can pass target_thread: nil
to enable tracing on all threads, to get the previous
default behavior.

Fixes [Bug #16889]

spec/ruby/core/tracepoint/enable_spec.rb

@@ -57,25 +57,50 @@ describe 'TracePoint#enable' do
       end.enable { event_name.should equal(:line) }
     end
 
-    it 'enables the trace object for any thread' do
-      threads = []
-      trace = TracePoint.new(:line) do |tp|
-        # Runs on purpose on any Thread
-        threads << Thread.current
-      end
-
-      thread = nil
-      trace.enable do
-        line_event = true
-        thread = Thread.new do
-          event_in_other_thread = true
+    ruby_version_is '3.2' do
+      it 'enables the trace object for any thread' do
+        threads = []
+        trace = TracePoint.new(:line) do |tp|
+          # Runs on purpose on any Thread
+          threads << Thread.current
         end
-        thread.join
-      end
 
-      threads = threads.uniq
-      threads.should.include?(Thread.current)
-      threads.should.include?(thread)
+        thread = nil
+        trace.enable do
+          line_event = true
+          thread = Thread.new do
+            event_in_other_thread = true
+          end
+          thread.join
+        end
+
+        threads = threads.uniq
+        threads.should.include?(Thread.current)
+        threads.should_not.include?(thread)
+      end
+    end
+
+    ruby_version_is ''...'3.2' do
+      it 'enables the trace object for any thread' do
+        threads = []
+        trace = TracePoint.new(:line) do |tp|
+          # Runs on purpose on any Thread
+          threads << Thread.current
+        end
+
+        thread = nil
+        trace.enable do
+          line_event = true
+          thread = Thread.new do
+            event_in_other_thread = true
+          end
+          thread.join
+        end
+
+        threads = threads.uniq
+        threads.should.include?(Thread.current)
+        threads.should.include?(thread)
+      end
     end
 
     it 'can accept arguments within a block but it should not yield arguments' do