Check that there is a single non-phantom field #763

Job	Run time
write-json-symtab-regression	21m 6s
regression (macos-11)	44m 9s
regression (ubuntu-20.04)	35m 5s
bookrunner	27m 47s
benchcomp-tests	7m 0s
perf	48m 26s
regression (ubuntu-22.04)	48m 51s
releasebundle-macos (macos-11)	8m 25s
releasebundle-ubuntu	36m 5s
	4h 36m 54s

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