Add tests for similarity functions

[kernelmethod]

Currently there are only tests for registering new similarities in test/test_similarities.jl. There should also be some tests to ensure that similarities are computed correctly in the first place.

[kernelmethod]

As of 3ff9c2f there are currently tests for the following similarities:

• l1, l2, and lp distance for Vectors
• L1, L2, and Lp distance for functions
• cossim for both Vectors and functions
• jaccard for Sets

This leaves the following similarities to be tested:

• inner_prod for both Vectors and functions
• wasserstein_1d for functions (although wasserstein_1d, wasserstein1_1d, and wasserstein2_1d have yet to be implemented).

[kernelmethod]

Tests for inner_prod were added in 5756eba.

[kernelmethod]

wasserstein_1d, wasserstein1_1d, and wasserstein2_1d were removed (at least tentatively) in 9ee87cb. I'm closing this issue for now, I'll reopen it if I end up adding more similarities to the package and they suffer the same problem.