Some tests create a so-called sandbox directory, use it for the files
the object under test creates, then remove that temporary sandbox
directory. To do this they use getTemporaryDir(). The test_jsonrpcserver
test attempted to do the same thing, but since that function wasn't
readily available, it used a similarly sounding
fs::temp_directory_path(). That function, however, just references /tmp.
As a result, test_jsonrpcserver would store files in /tmp then remove
/tmp instead of a test-specific temporary directory. This is a problem
since it would remove other processes' temporary files stored in /tmp
along with its own test files. This patch fixes this by creating
getTemporaryDir() for test_jsonrpcserver and calling that instead.

Fixes apache#8792