Merge #11849: [tests] Assert that only one NetworkThread exists
5c8ff26 [tests] Add NetworkThread assertions (John Newbery)
34e08b3 [tests] Fix network threading in functional tests (John Newbery)
74e64f2 [tests] Use network_thread_start() in tests. (John Newbery)
5fc6e71 [tests] Add network_thread_ utility functions. (John Newbery)
Pull request description:
Add assert that only one NetworkThread exists at any time in functional tests, and fix cases where that wasn't true.
fixes #11776
Tree-SHA512: fe5d1c59005f94bf66e11bb23ccf274b1cd9913741b56ea11dbcd21db4cc0b53b4413c0c4c16dbcd6ac611adad5e5cc2baaa39720598ce7b6393889945d06298