[C++] Write 64-bit integers as strings in JSON integration test files

[asfimport]

C++ side of ARROW-1875

Reporter: Wes McKinney / @wesm
Assignee: Ben Kietzman / @bkietz

Related issues:

• [Developer] Tracking issue: use strings for 64-bit int64 and uint64 in integration test JSON format (is a child of)

PRs and other links:

• GitHub Pull Request #5267
• GitHub Pull Request #6623

_{Note: This issue was originally created as ARROW-6310. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 5267
#5267