[C++] Expose Arrow's *FromJson, Assertion and Random generator helper functions

Helper functions would be useful when testing in projects that use Arrow. BatchesWithSchema, MakeBasicBatches, Finishes and ResultWith could be considered as well.

- FromJSON helpers (from `arrow/testing/gtest_util.h`)
- Assertion helpers (from `arrow/testing/gtest_util.h`), both macros (e.g., `ASSERT_OK_AND_ASSIGN`) and functions (e.g., `AssertArraysEqual`)
- Random generators (from `arrow/testing/random.h`)

**Reporter**: [Rok Mihevc](https://issues.apache.org/jira/browse/ARROW-18371) / @rok
**Assignee**: [Bryce Mecum](https://issues.apache.org/jira/browse/ARROW-18371) / @amoeba
**Watchers**: [Rok Mihevc](https://issues.apache.org/jira/browse/ARROW-18371) / @rok

_{**Note**: *This issue was originally created as [ARROW-18371](https://issues.apache.org/jira/browse/ARROW-18371). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}