Move tests from slow/kani back to regular suite (#4202)

These tests were moved while working towards
#2995, but do not appear to
be excessively slow even with the final version committed when
completing that PR. (`copy_empty_string_by_intrinsic.rs` completes
within a few seconds, `push_slow.rs` takes one minute and around 4 GB of
memory, but this is still well below what other tests in our regression
test suite take.)

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under the terms of the Apache 2.0 and MIT licenses.

Author: tautschnig