Add env var to unit test

Commit 5c46d1e6 added a unit test that depends on an environment
variable, but the test raises an exception when the environment
variable does not exist.

Addresses:

    3) Puppet::Type::Package::ProviderChocolatey when fetching a package list should query chocolatey
    Failure/Error: commands=ENV['ChocolateyInstall'] + "/chocolateyInstall/chocolatey.cmd"
    NoMethodError:
    undefined method `+' for nil:NilClass
    # ./spec/unit/chocolatey_spec.rb:94
Align rspec expectations with code

The package provider receives 3 arguments, so
the rspec tests now expect 3 arguments.

Resolve failures such as:

    unexpected invocation:
    Package[chocolatey](provider=chocolatey).chocolatey('install', 'chocolatey', nil)
    unsatisfied expectations: - expected exactly once, not yet invoked:
    Package[chocolatey](provider=chocolatey).chocolatey('install', 'chocolatey')
Fix rspec test for querying chocolatey

* Fix existing test
* Add extra test

resolves:

    1) Puppet::Type::Package::ProviderChocolatey when fetching
    a package list should query chocolatey
       Failure/Error: provider.instances
       MiniTest::Assertion:
         unexpected invocation: Puppet::Type::Package::ProviderChocolatey.execpipe([nil, ' version all -lo ^| % { \\\'{0}=={1}\\\' -f $_.Name, $_.Found }'])
         unsatisfied expectations:
         - expected exactly once, not yet invoked: Puppet::Type::Package::ProviderChocolatey.execpipe()
         satisfied expectations:
         - allowed any number of times, not yet invoked: Facter.value(:osfamily)
         - allowed any number of times, not yet invoked: Facter.value(:operatingsystem)
         - allowed any number of times, not yet invoked: Puppet::Type::Package::ProviderChocolatey.healthcheck(any_parameters)
       # ./lib/puppet/provider/package/chocolatey.rb:63:in `instances'
       # ./spec/unit/chocolatey_spec.rb:106:in `block (3 levels) in <top (required)>'
use common style for spec tests

* use 2-space soft tabs instead of hard tabs
* fix indents
* remove trailing whitespace