Fix path-dependent failures in assess-artifacts test (rust-lang#2849)

Shorten the tables frames in the `expected` file for the
`cargo-kani/assess-artifacts` test. Otherwise, the test will fail when
running the regression suite in a shorter-than-usual directory path
(e.g., `/tmp/kani`). Came across this problem this morning when
preparing for the release.

Author: adpaco-aws

tests/cargo-kani/assess-artifacts/expected

@@ -11,9 +11,9 @@ Analyzed 1 packages
  none (success)        |               2
  unsupported_construct |               2
 =========================================
-======================================================================================================
+==========================================
  Candidate for proof harness   | Location
--------------------------------+----------------------------------------------------------------------
+-------------------------------+----------
  a_supported_test_from_tests   |
  a_supported_test_from_the_lib |
-======================================================================================================
+==========================================