Currently, if expect_lint_free() fails, the message is

Not lint free

Which, while correct, isn't very helpful and requires another lintr run to find out where the lints are.
Instead, we should show the first couple of lints in the message, e.g.

Number of lints (42) not equal to 0.

[output of print.lints(head(lints, 10L))]

and 32 more...

Compare expect_equal(1:100, numeric()):

1:100 (actual) not equal to numeric() (expected).

actual: 1 2 3 4 5 6 7 8 9 10 and 90 more...
expected: ...