complete: Don't change to _GO_ROOTDIR

[mbland]

Closes #123. Tab completion will no longer change the working directory to _GO_ROOTDIR, the tradeoff being:

• The compspec specifies -o nospace now instead of -o filenames.
• This means that the ./go framework must add traling slashes to directory name completions, as well as add a space when returning a single completion (unless it ends with a /).
• Tab-completed paths now won't trim the directory prefix from multiple matches when completing into more than one directory.

To this last point, per @jeffkole, having longer completion paths seems less surprising and annoying than having the working directory change. I'd originally thought it would mean completing absolute paths, but it turns out that wasn't necessary after all. In fact, the completed path on the command line stays the same length as before this change; it's just the values displayed when multiple matches exist that are longer.

The core of the @go.compgen solution from lib/complete is pretty straightforward, but since ./go complete will now add trailing slashes for directories, and trailing spaces when there is only one match, a lot of tests needed very minor updates.

Another bonus of this solution: Previously I named the test directory tests because tab-completing the test command would cause the previous implementation to add a slash if a test directory existed. Since we're not setting -o filenames, and are now filtering filename completions through @go.compgen, this should not be an issue for other projects. The "doesn't add trailing slashes when not called with -d or -f" test case from tests/complete/compgen.bats covers this.

While touching all these test files, I decided to apply a few other convenient updates, namely:

• adding test_filter to setup
• Replacing IFS=$'\n' with the updated assert_{success,failure} that accept multiple arguments to check for line-by-line equality

[coveralls]

Coverage decreased (-0.03%) to 95.288% when pulling f52b6d8 on tab-comp into b103bd2 on master.