Add some downstream tests to be run by osci.tier0

This should be an equivalent to what system GCC of CentOS Stream 10
already has [1].

Because gcc-toolset-15-gcc will be scl-less and I have no prototype
to work with, let's take this as a first draft for now. Definitely
using the WITH_SCL variable is confusing and an obvious candidate
for an improvement...

Related: RHEL-92605

[1] 5780c4d2e3

.fmf/version

@@ -0,0 +1 @@
+1

plans/tier0.fmf

@@ -0,0 +1,36 @@
+# 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
+environment+:
+  WITH_SCL: gcc-toolset-15-env
+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/gcc.git
+    filter: 'tag: CI-Tier-1'
+provision+:
+  hardware:
+    disk:
+      - size: ">= 60 GiB"
+    memory: ">= 8 GiB"
+prepare+:
+  - how: shell
+    script:
+      - dnf config-manager --set-enabled '*CRB*' || true
+      - dnf config-manager --set-enabled crb || true
+      - dnf config-manager --set-enabled rhel-buildroot || true
+      - dnf config-manager --set-enabled epel || true
+execute:
+  how: tmt