`spago test` errors when there are no tests

[elliotdavies]

Running spago test on a project where there isn't a test directory results in this error:

Installation complete.
purs compile: No files found using pattern: test/**/*.purs
Build succeeded.
internal/modules/cjs/loader.js:670
    throw err;
    ^

Error: Cannot find module '../output/Test.Main'
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:668:15)
    at Function.Module._load (internal/modules/cjs/loader.js:591:27)
    at Module.require (internal/modules/cjs/loader.js:723:19)
    at require (internal/modules/cjs/helpers.js:14:16)
    at Object.<anonymous> (/home/elliot/<filepath elided>/.spago/run.js:3:1)
    at Module._compile (internal/modules/cjs/loader.js:816:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:827:10)
    at Module.load (internal/modules/cjs/loader.js:685:32)
    at Function.Module._load (internal/modules/cjs/loader.js:620:12)
    at Function.Module.runMain (internal/modules/cjs/loader.js:877:12)
spago: Tests failed: 1

I think this should probably either succeed (because in effect 0/0 tests passed) or fail in a nicer manner.

[f-f]

This is definitely good, but how would we detect this?

The implementation for spago test is just calling the Test.Main module with node, so to detect there's no such module we should somehow find out about all the modules in the included sources?
A low-tech approach would be full-text search on all the files searching for the string module Test.Main, but at that point we might as well just implement a parser.. (which we could also use for #165)