KVM: selftests: Create a new guest_memfd for each testcase

Refactor the guest_memfd selftest to improve test isolation by creating a
a new guest_memfd for each testcase.  Currently, the test reuses a single
guest_memfd instance for all testcases, and thus creates dependencies
between tests, e.g. not truncating folios from the guest_memfd instance
at the end of a test could lead to unexpected results (see the PUNCH_HOLE
purging that needs to done by in-flight the NUMA testcases[1]).

Invoke each test via a macro wrapper to create and close a guest_memfd
to cut down on the boilerplate copy+paste needed to create a test.

Link: https://lore.kernel.org/all/20250827175247.83322-10-shivankg@amd.com
Reported-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20251003232606.4070510-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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