test: use 'until' instead of 'while !'

test: use 'until' instead of 'while !'

In general, it's better to avoid a negation. And "!" is special, because it is
used for history expansion, i.e. the same command would behave differently if
pasted on the command line.

Inspired by 4a899c5a239eb50df3f596af4ff145f4a2d33f23.