Fix failing stats assert in test_rollback_to_stable10

unit-test failed on OS X 10.12

Host: macos-1012-53.macstadium.build.10gen.cc
Project: WiredTiger (develop)
Commit: diff: WT-7038 Update headers to be C++ compatible (#6205)

• Included wt_internal.h in cpp test_harness.h
• Moved a number of internal struct definitions outside of the struct they were declared in
• Added comments to new top level structs

Co-authored-by: Luke Pearson <luke.pearson@mongodb.com> | 12 Jan 21 03:48 UTC
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