tests/sys/opencrypto: fix whitespace per PEP8

Replace hard tabs with four-character indentations, per PEP8.

This is being done to separate stylistic changes from the tests from functional
ones, as I accidentally introduced a bug to the tests when I used four-space
indentation locally.

No functional change.

MFC after: 2 months
Approved by: emaste (mentor: implicit blanket approval for trivial fixes)