tests: add llvm-toolset-14.0 requirements to tests and plans

tests/build-gating.fmf

@@ -23,6 +23,10 @@ adjust:
     environment+:
       WITH_SCL: "scl enable llvm-toolset-13.0 rust-toolset-1.58"
     when: "collection == llvm-toolset-13.0"
+  - because: "When testing SCL-ized LLVM, the collection must be enabled first"
+    environment+:
+      WITH_SCL: "scl enable llvm-toolset-14.0"
+    when: "collection == llvm-toolset-14.0"
 
 discover:
     - name: lldb-tests

tests/python-embedded-interpreter/main.fmf

@@ -16,3 +16,8 @@ adjust:
     environment+:
       LLDB_PACKAGE: llvm-toolset-13.0-lldb
     when: "collection == llvm-toolset-13.0"
+  - require+:
+      - llvm-toolset-14.0-lldb
+    environment+:
+      LLDB_PACKAGE: llvm-toolset-14.0-lldb
+    when: "collection == llvm-toolset-14.0"