Create mock.nf in project folder instead of meta folder

[lukfor]

This PR introduces the creation of the mock.nf file in the project folder (instead of the meta folder) to preserve Nextflow's projectDir. The mock file has a unique name (.nf-test-${test-hash}.nf) and after execution, the file is moved to the meta folder for debugging purposes. Consequently, there's no longer a need to stage files to the meta directory.

This PR addresses the following issues:

• Automatic staging of assets bin and lib folders is broken (see: Automatic staging of assets bin and lib folders broken #193).
• projectDir references become invalid under nf-test (see: projectDir references become invalid under nf-test #99 (comment)).

Additionally, this change could resolve the issue:

• Process relative paths aren't resolved correctly (see: Process relative paths aren't resolved correctly #146).

[k1sauce]

This will fix an issue I've have with relative imports of modules, for example having something like this in a workflow

include { FASTQC } from "${projectDir}/modules/nf-core/fastqc"

Will not work in the meta dir when projectDir isn't resolved to the top level