Server: add more unit tests for expected server functionality

[camerondurham]

Some basic server unit tests exist here already: https://github.com/camerondurham/runner/blob/main/server/main_test.go

We don't need all of the tests below and can have different/more tests. This is just some basic test ideas to improve confidence in code changes that we're not breaking the server functionality.

• basic test for throwing 400 error function: https://github.com/camerondurham/runner/blob/main/server/main.go#L84
• basic tests that trigger runtime failure branch: https://github.com/camerondurham/runner/blob/main/server/main.go#L66-L69
• basic test that language endpoint has well formed response (just make sure we always serialize the response matching our api types): https://github.com/camerondurham/runner/blob/main/server/main.go#L23