junit: Fix CLI runs on individual files

[fmeum]

When all user-provided libFuzzer path arguments are files, the JUnit
executor now correctly starts libFuzzer in reproduction mode by not
adding any directories to the command line. In particular, this fixes
a crash when Jazzer is invoked on a single file to reproduce a
particular finding.

[ci-fuzz-check]

	1 open finding detected  View Findings
	The final number may be higher than displayed as the scan progresses in the background. For more information, check the Code Intelligence Documentation

[ci-fuzz-check]

	1 open finding detected  View Findings
	The final number may be higher than displayed as the scan progresses in the background. For more information, check the Code Intelligence Documentation

[ci-fuzz-check]

	1 open finding detected  View Findings
	The final number may be higher than displayed as the scan progresses in the background. For more information, check the Code Intelligence Documentation

[fmeum]

@br-lewis @bertschneider The finding is an NPE in Helpers#assertMutator, which makes me think that the fuzz test may not be executed with the experimental mutator. The experimental mutator would honor the @NotNull annotation, Autofuzz doesn't.

[ci-fuzz-check]

	1 open finding detected  View Findings
	The final number may be higher than displayed as the scan progresses in the background. For more information, check the Code Intelligence Documentation

[fmeum]

This is ready for review again, I fixed the Windows CI issue.

[ci-fuzz-check]

	1 open finding detected  View Findings
	The final number may be higher than displayed as the scan progresses in the background. For more information, check the Code Intelligence Documentation

[ci-fuzz-check]

	1 open finding detected  View Findings
	The final number may be higher than displayed as the scan progresses in the background. For more information, check the Code Intelligence Documentation

[bertschneider]

@fmeum Yes, you're right! Only the bazel tests used the experimental mutator. Due to the previous CI Sense integration problems we didn't notice the missing flag in cifuzz.yaml.

The experimental mutator can be enabled by adding the following snippet to cifuzz.yaml:

 engine-args:
  - --experimental_mutator

Do you want to enable the experimental mutator in this PR or should I create a new one?

[fmeum]

Will do, thanks for the pointer!

[bertschneider]

LGTM