TEST: explicitly require stringio

[mtasaka]

Since spec/spec_helper.rb uses StringIO class,
make this file explicitly require stringio.

[mtasaka]

Testing with

• ruby 3.4.0dev (2024-11-07 master df3395f2e3) +PRISM [x86_64-linux] ruby/ruby@df3395f

Executing bson-ruby head
e1f5d8f
rspec testsuite with ~/.spec removed generates the following error:

$ env RUBYLIB=$(pwd)/lib:$(pwd)/ext/bson/ rspec

An error occurred while loading ./spec/bson/array_spec.rb.
Failure/Error:
  class AlternateIO < StringIO
  
    # Read a byte from the stream.
    #
    # @returns [ String ] A String representation of the next byte.
    def readbyte
      super.chr
    end

NameError:
  uninitialized constant StringIO
# ./spec/spec_helper.rb:68:in '<top (required)>'
# ./spec/bson/array_spec.rb:16:in '<top (required)>'


Finished in 0.00055 seconds (files took 2.75 seconds to load)
0 examples, 0 failures, 1 error occurred outside of examples

With ~/.spec restored, the following error does not happen, perhaps because loading rspec format style somewhere loads stringio internally in rspec. But anyway I think explicitly adding stringio dependency inside bson-ruby is desirable.