Migrate testing to Go's testing package

[fujimotos]

While writing a unit test for #105, I noticed that https://github.com/bmizerany/assert has been abandoned (circa 2016).

The current preferred way to implement testing seems to be Go's testing package. Let's migrate fluent-logger-golang's testing to use it.

[wasim-nihal]

Hi, I would like to contribute here. Please find the PR for this change https://github.com/fluent/fluent-logger-golang/pull/121 and let me know if any changes are required.