Timeout secs can be float

please ignore commits https://github.com/LewisHaley/stb-tester/commit/c264a7d353e35ee01548874cb2efe4a08bb05b6d and https://github.com/LewisHaley/stb-tester/commit/a5230cbe834b43ded29035c4e43530d2b6b3b2e6,

this pull request is only meant to contain https://github.com/LewisHaley/stb-tester/commit/7d159b8e9c550302178ff255ab36b51cf8b92f3f.