improve reported errors in easyconfigs test suite

[Flamefire]

• Use the new return_conflicts parameter to show the conflicts in the error message instead of printing it to stderr during the status output when the test is run.
• Requires Add return_conflicts option to check_conflicts function and enhance tests easybuild-framework#5000
• Especially "binutils or GCC is a build dep: " is a misleading message when the test fails because it is not.

[Flamefire]

@boegel Can we get this in please? Found another instance where this would have helped: https://github.com/easybuilders/easybuild-easyconfigs/actions/runs/21939459431/job/63361191232?pr=25045

[boegel]

@Flamefire This needs another extra tiny change because of change in return value?

FAIL: test_conflicts (test.easyconfigs.easyconfigs.EasyConfigTest.test_conflicts)
Check whether any conflicts occur in software dependency graphs.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/easybuild-easyconfigs/easybuild-easyconfigs/test/easyconfigs/easyconfigs.py", line 331, in test_conflicts
    self.assertEqual(check_conflicts(self.ordered_specs, modules_tool(), check_inter_ec_conflicts=False,
AssertionError: [] != ''

Or maybe just assertFalse ?

[Flamefire]

Yes, should be an empty list

[boegel]

lgtm