Migrate DefaultReport from File to Path

[martingrossmann]

Description

Since using the updated libs of commons-io:commons-io (#426) there is an issue in generating the final report directory in a special case:
Running a test of a Gradle module with gradle submodule:test the user.dir is the project root but java.io.File creates files under <root>/submodule. Current FileUtils class is using java.nio.Path which tries to operate only the real user.dir.
Under Windows the report can be generated, but an old version cannot be deleted. Under Linux there is an exception at moving the report from temp to the final directory.

This PR migrates all file operations in DefaultReport to java.nio.Path. In case of running Gradle tests in submodules, the report will be placed in the real user.dir:

• gradle submodule:test --> <project root>/test-report
• cd submodule && gradle test --> <project root>/submodule/test-report

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules