(PDK-369) Improve error context for pdk test unit failures

[rodjek]

Adds the spec description to the output for rspec events, as well as up to 5 lines of context of the failing spec (up to 2 on either side of the failing line).

Example:

asmodean :0: pdk/foo (git:sdk-369 {2} U:1 ?:2!)$ ../bin/pdk test unit
[✔] Preparing to run the unit tests.
[✖] Running unit tests.
  Evaluated 5 tests in 0.206083208 seconds: 5 failures, 0 pending.
[✔] Cleaning up after running unit tests.
failed: rspec: ./spec/classes/foo_spec.rb:8: expected that the catalogue would not compile but it does
  foo on windows-2012-x64 should not compile into a catalogue without dependency cycles
  Failure/Error:
        let(:facts) { os_facts }
  
        it { is_expected.not_to compile }
      end
    end

failed: rspec: ./spec/classes/foo_spec.rb:8: expected that the catalogue would not compile but it does
  foo on debian-8-x86_64 should not compile into a catalogue without dependency cycles
  Failure/Error:
        let(:facts) { os_facts }
  
        it { is_expected.not_to compile }
      end
    end

failed: rspec: ./spec/classes/foo_spec.rb:8: expected that the catalogue would not compile but it does
  foo on windows-2012 R2-x64 should not compile into a catalogue without dependency cycles
  Failure/Error:
        let(:facts) { os_facts }
  
        it { is_expected.not_to compile }
      end
    end

failed: rspec: ./spec/classes/foo_spec.rb:8: expected that the catalogue would not compile but it does
  foo on ubuntu-16.04-x86_64 should not compile into a catalogue without dependency cycles
  Failure/Error:
        let(:facts) { os_facts }
  
        it { is_expected.not_to compile }
      end
    end

failed: rspec: ./spec/classes/foo_spec.rb:8: expected that the catalogue would not compile but it does
  foo on redhat-7-x86_64 should not compile into a catalogue without dependency cycles
  Failure/Error:
        let(:facts) { os_facts }
  
        it { is_expected.not_to compile }
      end
    end

[coveralls]

Coverage increased (+0.05%) to 94.121% when pulling 92217f1 on rodjek:sdk-369 into e7ec32a on puppetlabs:master.

[coveralls]

Coverage increased (+0.05%) to 94.121% when pulling ba96a75 on rodjek:sdk-369 into e7ec32a on puppetlabs:master.