Suppress warnings during testing

[ksss]

Suppress the following warnings during running rake stdlib_test.

rbs/lib/rbs/unit_test/convertibles.rb:9: warning: undefining '__id__' may cause serious problems
rbs/lib/rbs/unit_test/convertibles.rb:9: warning: redefining '__id__' may cause serious problems
rbs/lib/rbs/unit_test/spy.rb:41: warning: redefining '__send__' may cause serious problems
rbs/lib/rbs/unit_test/spy.rb:94: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)