x86/access: Pre-allocate all page tables at (sub)test init

Pre-allocate the page tables for each test instead of allocating page
tables on every. single. iteration.  In addition to being abysmally slow,
constantly allocating new page tables obliterates any hope of providing
meaningful test coverage for shadow paging, as using a new upper level
PTE for every iteration causes KVM to sync children, which prevents
exposing TLB flushing bugs in KVM.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-14-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>