fix easyconfig + extensions progress bars when --skip is used

[boegel]

Two problems being fixed here:

• Already installed extensions that were filtered out were still being counted to determine the total size of the extensions progress bar
  • => fixed in 8d1d8c6 by updating the total count after skipping already installed extensions
• Determining the total step count by checking which steps were being skipped didn't actually work, because the self.skip_step method actually only works after the check_readiness_step method has been run
  • => fixed in fe9b9b1 by not trying to exclude skipped steps when determining the size of the easyconfigs progress bar, but always updating it after a step was run or skipped

[branfosj]

Fairly minor change as skippable is no longer user.

And the tests are hitting that download issue we've been seeing today :(

[boegel]

And the tests are hitting that download issue we've been seeing today :(

Fix for that annoying test in #3883, hopefully relying on sources.easybuild.io will prove to be more stable than https://ftp.gnu.org...