Liters should be labeled as SI units

[nickwallen]

Liters (and all other permutations of) are not currently labeled as part of the SI/metric system. They simply return an empty string; see this example. This PR fixes that.

Changes

• Added SI UnitOptionSystem for Liters.
• Added a test case to validate the 'system' for all volume units. Let me know if I missed a more logical location for these tests. I can alter as you see fit.
• Added github.com/stretchr/testify as a dependency for the test case. I would understand if you do not want to add this net-new dependency for your project. I can alter as you see fit.

[bcicen]

merged; thanks @nickwallen !

testify works well and will be of use when testing is expanded in the future; we can keep the dep in go.mod as well.