[checks] Collapse clauses for described conditions

`IterableChecks.any`, `IterableChecks.every`, `MapChecks.containsKeyThat`,
and `MapChecks.containsValueThat` embed the description of a condition.
They collapse to a single line at the root, through a predicate noun such as
`an iterable containing a value that is greater than <7>`, but their clause
always used the multi-line form of a label followed by the indented
description. Where the clause itself is shown, such as when one of these
expectations is nested inside the condition of another, the description
never collapsed, while the same check reached through `has` did. Reported in
https://github.com/dart-lang/test/pull/2784#discussion_r4230540680:

    check([[1]]).any(.it()..any(.it()..isGreaterThan(1)));

    Expected: a List<List<int>> that:
      contains a value that:
        contains a value that:
          is greater than <1>

Add an internal `describedClause` helper next to `describedPredicateNoun`.
It joins the label to the description of the condition on a single line when
the condition describes in a single line and the result fits within 80
columns, and otherwise keeps the label followed by the indented description.
Both helpers share the logic for joining and the length limit. Use it for
the clauses of all four expectations. The inner `any` above now describes
itself as a single line, so the outer `any` collapses to a single
`Expected:` line at the root:

    an iterable containing a value that contains a value that is
    greater than <1>

Joining the label of `every`, `only has values that`, to a description such
as `is less than <10>` reads as `only has values that is less than <10>`.
Describe the condition as `every element` instead, which reads as
`every element is less than <10>` on one line, and `every element:` followed
by the description otherwise. Change the predicate noun to match:
`an iterable where every element is less than <10>`.