CDRIVER-3008 fix possible leak in run_json_general_test

[kevinAlbs]

Summary

• fix possible leak in run_json_general_test
• document example use of MONGOC_JSON_SUBTEST

Background & Motivation

CDRIVER-3008 includes the reported leak discovered by Coverity. The leak can be reproduced with:

export MONGOC_JSON_SUBTEST="Aggregate with deterministic encryption"
./cmake-build/src/libmongoc/test-libmongoc -d --no-fork -l "/client_side_encryption/legacy/aggregate"

LSan reports:

==52500==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 40 byte(s) in 1 object(s) allocated from:
    #0 0x102f515b0 in wrap_malloc+0x8c (libclang_rt.asan_osx_dynamic.dylib:arm64+0x515b0) (BuildId: 2d3005610c2a36e7a5269cbe22ab009f32000000200000000100000000000b00)
    #1 0x100fb4c90 in bson_malloc bson-memory.c:100
    #2 0x100fd6508 in bson_strdup bson-string.c:331
    #3 0x10087c818 in _mongoc_getenv mongoc-util.c:683
    #4 0x100a13f08 in test_framework_getenv test-libmongoc.c:264
    #5 0x1009e4890 in run_json_general_test json-test.c:1819
    #6 0x100f18840 in test_client_side_encryption_cb test-mongoc-client-side-encryption.c:108
    #7 0x100eb3e7c in TestSuite_RunTest TestSuite.c:648
    #8 0x100eb19dc in TestSuite_RunAll TestSuite.c:1094
    #9 0x100eb0888 in TestSuite_Run TestSuite.c:1139
    #10 0x100607680 in main test-libmongoc-main.c:157
    #11 0x101df1088 in start+0x204 (dyld:arm64+0x5088) (BuildId: a2ee361189123e1dbf8bfed54862d4c932000000200000000100000000060c00)
    #12 0x902efffffffffffc  (<unknown module>)

[joshbsiegel]

LGTM