Use a stub for $stdin during testing

[mattbrictson]

Summary

This allows the tests to run even in a situation where stdin isn't available, for example when the test is run with < /dev/null as is the case for some packaging scripts.

Fixes #2032

Short checklist

• Did you run bundle exec rubocop -a to fix linter issues?
• If relevant, did you create a test?
• Did you confirm that the RSpec tests pass?
• If you are fixing a bug or introducing a new feature, did you add a CHANGELOG entry? (this is not a user-facing fix)

[capistrano-bot]

	1 Warning
⚠️	Please update CHANGELOG.md with a description of your changes. If this PR is not a user-facing change (e.g. just refactoring), you can disregard this.

Here's an example of a CHANGELOG.md entry (place it immediately under the * Your contribution here! line):

* [#2033](https://github.com/capistrano/capistrano/pull/2033): Use a stub for $stdin during testing - [@mattbrictson](https://github.com/mattbrictson)

Generated by 🚫 Danger

[mattbrictson]

This fix is incomplete. There are still 2 failing tests when I run the tests with < /dev/null:

  1) Capistrano::DSL asking for a variable variable is provided sets the input as the variable
     Failure/Error: expect(dsl.fetch(:scm)).to eq "git"

       expected: "git"
            got: :svn

       (compared using ==)

       Diff:
       @@ -1,2 +1,2 @@
       -"git"
       +:svn
     # ./spec/integration/dsl_spec.rb:370:in `block (4 levels) in <top (required)>'

  2) Capistrano::DSL asking for a variable variable is not provided sets the variable as the default
     Failure/Error: $stdin.expects(:gets).returns("")

     Mocha::ExpectationError:
       not all expectations were satisfied
       unsatisfied expectations:
       - expected exactly once, not yet invoked: #<IO:0x108a28e20>.gets(any_parameters)
       satisfied expectations:
       - allowed any number of times, invoked once: #<IO:0x108a28d80>.print(any_parameters)
     # ./spec/integration/dsl_spec.rb:376:in `block (4 levels) in <top (required)>'

[mattbrictson]

All fixed now