Allowing cwd to be passed to testem without being overridden by ember-cli

[arthirm]

When ember-cli invokes testem it provides defaultOptions for testem to use as a fallback option. But currently, the fallback option from ember-cli is used as progOptions in testem. And since progOptions has higher priority than fileOptions, the cwd being set in the testem.js file of an ember app is being overridden by ember-cli's fallbackOption.

Solution is to add defaultOptions as a sibling to progOptions and fileOptions in testem. And ember-cli can set defaultOptions invoke testem.

Related work in testem is in the below PR.
testem/testem#1219

Previously this commit caused the tests to run in only 1 partition because when ember-cli was passing progOptions to testem, it didnt transform the option's property names like testPage to test_page which has partition information from ember exam. Hence by default it ran in 1 partition.
ember exam does not fail if it is asked to run in multiple partitions but it ran only in 1 partition.

[stefanpenner]

this LGTM. But I would like either @rwjblue or @Turbo87 to also sign-off, as last time I missed the issue which resulted in the regression.

[Turbo87]

haven't had time yet to test it, but if we land it I would prefer to have it go through the usual beta cycle to avoid issues like last time 🤔

[stefanpenner]

I'm good with beta.