Ensure test library issues json string line-by-line

[raoulstrackx]

#108659 introduces a custom test display implementation. It does so by using libtest to output json. The stdout is read line by line and parsed. The code trims the line read and checks whether it starts with a { and ends with a }.

Unfortunately, there is a race condition in how json data is written to stdout. The write_message function calls self.out.write_all repeatedly to write a buffer that contains (partial) json data, or a new line. There is no lock around the self.out.write_all functions. Similarly, the write_message function itself is called with only partial json data. As these functions are called from concurrent threads, this may result in json data ending up on the same stdout line. This PR avoids this by buffering the complete json data before issuing a single self.out.write_all.

(#109484 implemented a partial fix for this issue; it only avoids that failed json parsing would result in a panic.)

cc: @jethrogb, @pietroalbini