test_broken_bignum: use assert_separately instead of fork + waitpid2

The raw fork + Process.waitpid2 pattern causes a SEGV on i686-linux in
ruby/ruby CI. The crash occurs in waitpid_blocking_no_SIGCHLD going
through the 32-bit vdso, which is a Ruby VM / kernel interaction issue
unrelated to the json gem itself.

Using assert_separately avoids this by spawning a subprocess via
IO.popen instead of raw fork + waitpid2, and is also the idiomatic
pattern for running test code in an isolated process within ruby/ruby's
test framework.

Author: k0kubun

test/json/json_generator_test.rb

@@ -476,24 +476,19 @@ def foo.to_h
   end
 
   def test_broken_bignum # [ruby-core:38867]
-    pid = fork do
+    # Run in a separate process to avoid modifying core class of the parent
+    # process and introducing race conditions when tests are run in parallel.
+    assert_separately([], <<~RUBY)
+      require 'json'
       x = 1 << 64
       x.class.class_eval do
         def to_s
         end
       end
-      begin
+      assert_raise(TypeError) do
         JSON::Ext::Generator::State.new.generate(x)
-        exit 1
-      rescue TypeError
-        exit 0
       end
-    end
-    _, status = Process.waitpid2(pid)
-    assert status.success?
-  rescue NotImplementedError
-    # forking to avoid modifying core class of a parent process and
-    # introducing race conditions of tests are run in parallel
+    RUBY
   end
 
   def test_hash_likeness_set_symbol