add tests for release provider testing --all and --relnames options

[TheGeorge]

Tests for change proposed in erlang/rebar3#2630

+ shelltest -c --diff --all --execdir releases_test/releases.test
:releases_test/releases.test:1: [OK]
:releases_test/releases.test:2: [OK]
:releases_test/releases.test:3: [OK]
:releases_test/releases.test:4: [OK]

         Test Cases  Total
 Passed  4           4
 Failed  0           0
 Total   4           4

A local test succeeds on those tests

[tsloughter]

Looks good but just to be sure can you add a test to verify the release is built, like:

ls _build/default/rel/app1/releases
>>>= 0

[TheGeorge]

Ah yes, makes a lot of sense!

[tsloughter]

Should add a check that the output includes the expected error message:

Must specify the name of the release to build when there are multiple releases in the config