Warn and return when `node_tests` are empty

[alexanderjeurissen]

Context

When working on #103 an edge case was observed where a pattern (or list in #103) resulting allocator.node_tests when smaller then the number of CI_NODES results in 1-n nodes executing the complete test suite.

relevant line in rspec runner (bug applies to all runners):

https://github.com/KnapsackPro/knapsack/blob/d9abb662f6a5d5473c34af4d63ccdd077ee14935/lib/knapsack/runners/rspec_runner.rb#L15

This interpolated string, with an empty node_tests array results in the following command being executed (assuming a test-dir of spec and no additional args):

bundle exec rspec --default-path spec --

This behaves the same as bundle exec rspec --default-path spec which executes all specs.

Solution

This PR adds a guard clause to all runners to return whenever the node_tests is empty for a given node. This seems to be the desired behavior: there are no specs left to execute for this node, so do nothing.

[alexanderjeurissen]

@ArturT could you review this ?

[ArturT]

Hi @alexanderjeurissen

Note for myself to review it later:

It looks like this PR depends on the source code from the https://github.com/KnapsackPro/knapsack/pull/103#issuecomment-821987956 We need to decide first what to do with that PR before merging this PR.