Fix skipping tests on el8

pytest's -k flag doesn't accept `"not ()"` as an expression.

python-trustme.spec

@@ -18,13 +18,6 @@ which nobody trusts.  But you can trust it.  Trust me.}
 
 %bcond_without  tests
 
-%if %{defined el8}
-# The upstream test suite uses cryptography's rfc4514_string method, which
-# wasn't added until version 2.5.  RHEL 8 currently only provides version 2.3.
-# https://cryptography.io/en/latest/changelog/?highlight=rfc4514_string#v2-5
-%global skip_list test_ca_custom_names or test_issue_cert_custom_names
-%endif
-
 
 Name:           python-%{pkgname}
 Version:        0.6.0
@@ -67,7 +60,15 @@ rm -rf %{eggname}.egg-info
 
 %if %{with tests}
 %check
-%pytest --verbose -k "not (%{?skip_list})"
+%if %{defined el8}
+# The upstream test suite uses cryptography's rfc4514_string method, which
+# wasn't added until version 2.5.  RHEL 8 currently only provides version 2.3.
+# https://cryptography.io/en/latest/changelog/?highlight=rfc4514_string#v2-5
+%pytest --verbose -k "not (test_ca_custom_names or test_issue_cert_custom_names)"
+%else
+%pytest --verbose
+%endif
+
 %endif