tests: record HW requirements

This was originally stored in provision.fmf, but after the TMT
transition, this information should be preserved. It is probably not
honoured by that many CI systems, but things will get better over time.

See https://tmt.readthedocs.io/en/stable/spec/plans.html#hardware for
specification details.

tests/default.fmf

@@ -7,3 +7,6 @@ prepare:
     - name: packages
       how: install
       package: llvm
+provision:
+  hardware:
+    memory: ">= 4 GiB"