bigfile/tests/thread: Don't invalidate exactly same page to test general virtmem deadlock on loadblk

The main deadlock described in f49c11a3 (bigfile/virtmem: Do loadblk()
with virtmem lock released) (V,Z in T1; Z,V in T2) can happen if in T2 V
is taken for whatever reason - e.g. for invalidating completely
unrelated page to what is being loaded in T1.

For invalidation of the same page we have explicit separate
test_thread_load_vs_invalidate() which verifies how loadblk handles this
situation.

This patch prepares general test_thread_lock_vs_virtmem_lock() to also
test V vs Z deadlock for storeblk() case - when it will be called with
virtmem lock released: for storeblk it will be forbidden by virtmem
rules to invalidate pages of fileh for which writeout is in progress.

Updates: nexedi/wendelin.core#6