Run tests recursively

[nicolaferraro]

Part of #55, depends on #57

See also #59 and #60

We should add the possibility to run tests recursively in sub-dirs.

A test group can contain the following config:

config:
  recursive: true

When it's enabled (I'm not sure about the default value for it):

• .feature files present in the dir are still treated as tests belonging to the group and run
• sub-directories are treated as sub-test-groups to run (i.e. we should run them before or after the standard tests, using their own yaks-config.yaml config file).

Possible usages.

Basic

• dir1 (recursive)
  • test1.feature
  • dir2
    • test2.feature
    • test3.feature

Execution is: test1, test2, test3 in the current ns (not deleted after run).

SingleTestNS

• dir1 (recursive, temp-ns)
  • test1.feature
  • dir2
    • test2.feature
    • test3.feature

Execution is: test1, test2, test3 in a temporary namespace, deleted after run

MultipleTestNS

• dir1 (recursive, temp-ns)
  • test1.feature
  • dir2 (temp-ns)
    • test2.feature
    • test3.feature
  • dir3
    • test4.feature

Execution is:

• test1 in tempns1
• test2, test3 in tempns2
• test4 in tempns1 (same as test1)

Both temp ns deleted after run.