The name toSecsSinceEpoch() gave no hint to the fact that the test
was, in fact, *also* testing round-tripping of the ISODateFormat.
Since there are other tests for string conversion, make this a simple
test of toSecsSinceEpoch().
It did the round-tripping via two methods with overly-terse names that
might just as well be local lambdas - now redundant, so removed.
Change-Id: I1e4fb1cc90224312c995596a8f3fe2bc5d9dfa15
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
b1cf188e83