Run Cucumber tests if there is not other Instrumented tests included

[piotradamczyk5]

Author the user story for this feature

As a User, I want to run Cucumber tests, even I do not have another Instrumented tests in my test suite so I can run tests with a Cucumber-only test suite using Flank.

Is your feature request related to a problem? Please describe.
For now, Gcloud could run tests even apk contains Cucumber only tests.
Flank fast fail with the message No tests to run (disabling fast fail do not fix this situation)

Describe the solution you'd like
Provide additional flag ex. only-cucumber-tests which indicate that the new Matrix should be created

Describe alternatives you've considered
0. Described solution

1. Print a warning message that other tests should be included
2. Always create a matrix and try to run tests even for empty apks
3. Leave it as it is with information about this in FAQ section of Flank

Additional context
See PR connected to issue #1118 for more information

@bootstraponline @jan-gogo @pawelpasterz @adamfilipow92 @Sloox @axelzuziak-gogo
WDYT?

[bootstraponline]

@piotradamczyk5 the existing disable sharding flag should allow Flank to run cucumber tests.

[bootstraponline]

If the flag is enabled then we should skip the test count check.

[bootstraponline]

@piotradamczyk5 as Cucumber is not officially supported by FTL I think we can pause work on this in favor of higher priority tickets. Thanks for the investigation and updating the readme.