Configure tier0 testing

Based on the component's tier1 tests in
  https://gitlab.com/redhat/centos-stream/tests/
Aligns with the rest of rhel-pt-gcc's components.

Related: RHEL-114068

plans/ci.fmf

@@ -1,5 +0,0 @@
-summary: CI Gating Plan
-discover:
-    how: fmf
-execute:
-    how: tmt

plans/tier0.fmf

@@ -0,0 +1,29 @@
+# Tier 0 test plan. It's actually Tier 1
+#   - limited to its public part
+#   - executed on the architecture at hand
+# When run by osci.tier0 on CentOS Stream or RHEL, it receives the following
+# context:
+#   arch=x86_64 (the architecture at hand, currently OSCI uses just x86_64)
+#   distro=rhel<M>.<m> (even for CentOS Stream builds)
+#   trigger=build
+
+summary: Tier 0 test plan
+discover:
+  - name: collect_info
+    how: shell
+    tests:
+      - name: /info/rpms
+        test: rpm -qa --last
+  - name: public_tests
+    how: fmf
+    url: https://gitlab.com/redhat/centos-stream/tests/libmpc.git
+    filter: tier:1
+execute:
+  how: tmt
+adjust+:
+  - when: distro > rhel-7
+    prepare+:
+      - how: shell
+        script:
+          - dnf config-manager --set-enabled '*CRB*' || true
+          - dnf config-manager --set-enabled crb || true