fix(compute/init): Init no longer fails if directory of same name as starter kit exists in current directory

[harmony7]

Resolves #1255.

This PR resolves the issue by placing the temporary file created during the download performed by fastly compute init --from in a temporary directory rather than the current directory.

Notes:

• This PR adds a setupSteps member to the test table in init_test, and runs those steps as part of the initialization of each test.

• This PR adds a few new test cases to init_test that uses the above setupSteps mechanism to create a directory with the same name as the starter kit before calling fastly compute init.

• The existing defer cleanup done to remove the temporary file has been removed, because the cleanup code for the temporary directory will clean it up.

[kpfleming]

Some minor Go style changes, but otherwise this looks great!

[harmony7]

Thanks for your review! I've applied the changes from your suggestions, please review again. Go ahead and merge it if you approve it as I will be away for the next few days.

[harmony7]

This pattern occurred many more times in this file so I applied it there too:

-spinErr := spinner.StopFail()
-if spinErr != nil {
+if spinErr := spinner.StopFail(); spinErr != nil {

(We may want to consider factoring this out)

[harmony7]

I had to revert the inlined assign/compare pattern for a couple of the cases because the variables that were assigned needed to be available later, outside the if block. I don't know if there's a Go-style best practice for this kind of pattern.

[kpfleming]

There is... use var <name> <type> before the if block, so that the declaration becomes independent of the block. It's not a big deal though.