Add unit tests for JSONRPC

[wheatear-dev]

Further to #43440, this PR contains several unit tests for the JSONRPC module.

1. Various invalid (i.e. not Dictionary or Array) derived classes of Variant produce a well-formatted error when given to JSONRPC::process_action
2. Likewise for JSONRPC::process_string
3. Using the given TestClassJSONRPC class, and its public method something (that appends ", please" to the input String):
   1. A Dictionary works correctly
   2. An Array has each of its inner Dictionary elements iterated over
   3. Giving a bad method name produces a well-formatted error.

Similarly to #89021, DOCTEST_CONFIG_NO_EXCEPTIONS_BUT_WITH_ALL_ASSERTS was added to the CPPDEFINES of the JSONRPC module, if both tests are enabled and exceptions are disabled, so that the REQUIRE assert could be used.

[Calinou]

Tested locally (rebased on top of master 83b916b), it works as expected. Code looks good to me.

Note that there's another pull request adding JSONRPC unit tests: #80480

[akien-mga]

Thanks!