remote add sets new endpoint as default, suppressible via flag

[preminger]

Description of the Pull Request (PR):

When adding a remote via singularity remote add <remote_name> <remote_uri>, the newly-added remote will be selected as the current remote endpoint by default.

This can be suppressed (and thus, returned to the previous behavior of remote add) by supplying the -n/--no-default flag to remote add.

This fixes or addresses the following GitHub issues:

• Fixes automatically select argument to remote add as current remote #1895

[dtrudg]

Needs e2e testing in e2e/remote/remote.go

The unit tests against remoteAdd() are using a positive 'makeDefault'.

Since the CLI behaviour is the opposite, a negative 'not- default', an e2e test is warranted.

The e2e remote tests are poorly structured, and it might be a bit of a pain to add these checks, but we should take this opportunity to improve the coverage of the CLI.

[preminger]

The unit tests against remoteAdd() are using a positive 'makeDefault'.

Wait, are we looking at the same thing here? I doubled the set of tests in TestRemoteAdd() in internal/app/singularity/remote_add_test.go - instead of 30, there are now 60 - and the two blocks differ in precisely the value of tt.makeDefault they have.

(I think it's possible that GitHub didn't show this because of its "large commits are hidden by default" thing.)

Plus, TestRemoteRemove() in internal/app/singularity/remote_remove_test.go also tests both settings of makeDefault now.