[C++] Move JSON testing code for integration tests to libarrow_testing

[asfimport]

This code contributes over 700KB to release builds and is never used

-rw------- 1 wesm wesm  34104 Jun 27 11:14 dictionary.cc.o
-rw------- 1 wesm wesm 199592 Jun 27 11:14 feather.cc.o
-rw------- 1 wesm wesm  63448 Jun 27 11:14 json_integration.cc.o
-rw------- 1 wesm wesm 727336 Jun 27 11:14 json_internal.cc.o
-rw------- 1 wesm wesm 828056 Jun 27 11:14 json_simple.cc.o
-rw------- 1 wesm wesm 185344 Jun 27 11:14 message.cc.o
-rw------- 1 wesm wesm 223592 Jun 27 11:14 metadata_internal.cc.o
-rw------- 1 wesm wesm   3416 Jun 27 11:14 options.cc.o
-rw------- 1 wesm wesm 557960 Jun 27 11:14 reader.cc.o
-rw------- 1 wesm wesm 285744 Jun 27 11:14 writer.cc.o

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #7557

_{Note: This issue was originally created as ARROW-9251. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 7557
#7557