DurationTest.Arithmetic performs inexact float calculation in test

DurationTest.Arithmetic does a calculation with not exactly representable floating point values and also performs an equality check,

EXPECT_EQ(Duration::create(3.3).get(), Seconds(10) * 0.33);

Here neither the value 3.3 nor 0.33 cannot be represented exactly as a floating point number so the check might fail incorrectly (as it does e.g. when compiling and executing the test under 32-bit on Debian8).

Instead we should just use exactly representable values to make sure the test will succeed as long as the implementation behaves as expected.