Resolve "Follow-up from "Skylink HEAD Request""

MERGE REQUEST

Overview

This MR cleans up skynet_test.go a bit because it has grown into a relatively large test suite. I moved some of the helpers we use in the renter package (Subtest etc) to the helpers file and adjusted some of the existing tests. The most important change is that test groups are reused now, thus saving valuable resources.

Example for Visual changes (ie Screenshot)

Before:

--- PASS: TestSkynet (27.63s)
--- PASS: TestSkynetSubDirDownload (43.33s)
--- PASS: TestSkynetDisableForce (60.83s)
--- PASS: TestSkynetMultipartUpload (82.80s)
--- PASS: TestSkynetNoFilename (99.28s)
--- PASS: TestSkynetBlacklist (148.51s)

After:

--- PASS: TestSkynet (41.42s)
    --- PASS: TestSkynet/TestSkynetBasic (9.00s)
    --- PASS: TestSkynet/TestSkynetMultipartUpload (4.47s)
    --- PASS: TestSkynet/TestSkynetNoFilename (0.90s)
    --- PASS: TestSkynet/TestSkynetSubDirDownload (2.36s)
    --- PASS: TestSkynet/TestSkynetDisableForce (1.47s)
    --- PASS: TestSkynet/TestSkynetBlacklist (5.19s)

Issues Closed

Closes #3875 (closed)

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods, or updating methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to work flow
• Issue added to Sia-UI repo for new supporting features
• Changelog updated