Support Collection GCC in the all_tests plan

Take care of installing the respective Collection and enabling it
in runtime.

Notes:

1. The script taking care of the installation part is missing. It's
   not relevant in Fedora anyway. I'll handle it later when needed
   for RHEL.

2. Getting the actual Collection from TMT context and using it
   (e.g. setting the WITH_SCL variable) isn't pretty. Unfortunately
   TMT allows using Context only in conditions, so... :-(

plans/all_tests.fmf

@@ -4,8 +4,45 @@ discover:
 execute:
     how: tmt
 adjust:
-    prepare+:
-        - name: Add secondary architecture RPMs
-          how: shell
-          script: ./plans/provide_secondary_arch_rpms.sh
-    when: distro == fedora and arch == x86_64
+    - prepare+:
+          - name: Add secondary architecture RPMs
+            how: shell
+            script: ./plans/provide_secondary_arch_rpms.sh
+      when: distro == fedora and arch == x86_64
+    - environment+:
+          COLLECTIONS: devtoolset-11
+          WITH_SCL: "scl enable devtoolset-11"
+      when: collection == devtoolset-11
+    - environment+:
+          COLLECTIONS: devtoolset-12
+          WITH_SCL: "scl enable devtoolset-12"
+      when: collection == devtoolset-12
+    - environment+:
+          COLLECTIONS: devtoolset-13
+          WITH_SCL: "scl enable devtoolset-12"
+      when: collection == devtoolset-13
+    - environment+:
+          COLLECTIONS: gcc-toolset-11
+          WITH_SCL: "scl enable gcc-toolset-11"
+      when: collection == gcc-toolset-11
+    - environment+:
+          COLLECTIONS: gcc-toolset-12
+          WITH_SCL: "scl enable gcc-toolset-12"
+      when: collection == gcc-toolset-12
+    - environment+:
+          COLLECTIONS: gcc-toolset-13
+          WITH_SCL: "scl enable gcc-toolset-13"
+      when: collection == gcc-toolset-13
+    - environment+:
+          COLLECTIONS: gcc-toolset-14
+          WITH_SCL: "scl enable gcc-toolset-14"
+      when: collection == gcc-toolset-14
+    - environment+:
+          COLLECTIONS: gcc-toolset-15
+          WITH_SCL: "scl enable gcc-toolset-15"
+      when: collection == gcc-toolset-15
+    - prepare+:
+          - name: Install Collection gcc
+            how: shell
+            script: ./plans/install_toolset.sh
+      when: distro != fedora and collection is defined