Change flag to --test-fail-when-no-test-suites.

This changes the default behaviour to succeed when no test suites are found. The
old behaviour can be achieved by passing --test-fail-when-no-test-suites.

Sessions with new changes:

$ cabal v2-test
Resolving dependencies...
No tests to run for the package initsample-0.1.0.0

$ cabal v2-test --test-fail-when-no-test-suites
Resolving dependencies...
cabal: Cannot run tests for the target '' which refers to the package
initsample-0.1.0.0 because it does not contain any test suites.