Some of the builds are like: "x64-mswin64_140" so the test should not assume 32bit.
It seems to me that this gem (first migrated from the old “ruby-decimal” gem) in 2014, may still be assuming that a rounding error occurs specifically on Windows 32bit platform, … Ie, 64bit Windows running 64 bit Ruby may not have the issue this gem is trying to detect.
Ie, see it’s closed issue Incorrectly rounded trigonometry results · Issue #3 · jgoizueta/flt · GitHub
So, my first idea of checking only the platform, may not be a good test.
But I still think that sniffing at the RUBY_PLATFORM string alone could be problematic.
Probably it would be better to run a test similar to the one the gem author ran in the issue #3 linked above. (There is a "test" folder included with the gem having quite a few test scenarios.)