If you run:

oe-selftest -r runtime_test.TestImage.test_testimage_dnf

with DISTRO unset (i.e. oe-core's nodistro), the test will fail:

Traceback (most recent call last):
  File "/media/build/poky/meta/lib/oeqa/core/decorator/__init__.py", line 35, in wrapped_f
    return func(*args, **kwargs)
  File "/media/build/poky/meta/lib/oeqa/core/decorator/__init__.py", line 35, in wrapped_f
    return func(*args, **kwargs)
  File "/media/build/poky/build-st-2173771/meta-selftest/lib/oeqa/runtime/cases/dnf_runtime.py", line 49, in test_verify_package_feeds
    self.assertTrue(matchobj is not None, msg = "Could not find the amount of packages in dnf repoinfo output: %s" %(output_repoinfo))
AssertionError: False is not true : Could not find the amount of packages in dnf repoinfo output: DNF version: 4.14.0
cachedir: /var/cache/dnf
OE Remote Repo:                                  17 kB/s | 520  B     00:00    
OE Remote Repo:                                 0.0  B/s |   0  B     00:00    
Errors during downloading metadata for repository 'oe-remote-repo':
  - Curl error (37): Couldn't read a file:// file for file:///etc/pki/packagefeed-gpg/PACKAGEFEED-GPG-KEY-nodistro-None [Couldn't open file /etc/pki/packagefeed-gpg/PACKAGEFEED-GPG-KEY-nodistro-None]
Error: Failed to retrieve GPG key for repo 'oe-remote-repo': Curl error (37): Couldn't read a file:// file for file:///etc/pki/packagefeed-gpg/PACKAGEFEED-GPG-KEY-nodistro-None [Couldn't open file /etc/pki/packagefeed-gpg/PACKAGEFEED-GPG-KEY-nodistro-None]
Ignoring repositories: oe-remote-repo
Total packages: 0

if you use DISTRO=poky, this doesn't happen.