IonRoundtripConcurrencyStressTest should include struct edge cases

[jobarr-amzn]

In light of the bugs addressed in:

• Fixes a bug that caused concurrent field access of cloned, read-only structs with more than 5 members to be non-deterministic. #630
• Fixes a bug that made concurrent access of a large nested IonStruct unsafe when only its parent had been made read-only. #722

We should augment the existing IonRoundtripConcurrencyStressTest to exercise IonStruct values with a) more than 5 fields, and b) child IonStructs with more than 5 fields. Better yet if the tests use STRUCT_INITIAL_SIZE instead of 5..

[jobarr-amzn]

This is redundant given the tests added in #722, see CloneTest.

IonRoundtripConcurrencyStressTest validates the independence of Ion readers/writers.