$  ./mach jstests --run-slow-tests  non262/extensions/clone-complex-object.js 
## non262/extensions/clone-complex-object.js: rc = 3, run time = 28.138113
/home/matthew/unified/js/src/tests/non262/extensions/clone-complex-object.js:105:17 Error: Assertion failed: got true, expected false: obj[1][0]
Stack:
  compareProperties@/home/matthew/unified/js/src/tests/non262/extensions/clone-complex-object.js:105:17
  isClone@/home/matthew/unified/js/src/tests/non262/extensions/clone-complex-object.js:143:30
  check@/home/matthew/unified/js/src/tests/non262/extensions/clone-complex-object.js:155:14
  checkSimpleIdentity@/home/matthew/unified/js/src/tests/non262/extensions/clone-complex-object.js:206:9
  @/home/matthew/unified/js/src/tests/non262/extensions/clone-complex-object.js:224:20
REGRESSION - non262/extensions/clone-complex-object.js
[0|1|0|0] 100% ==========================================================>|  28.6s
REGRESSIONS
    non262/extensions/clone-complex-object.js
FAIL

Presumably this started happening sometime in the last 5 years since this test was marked slow. The failing line is assertEq(da.configurable, shouldBeConfigurable(ca, aName), path2); so my bet is that there's something spec-wise which has recently changed causing this test to need being updated, but it was missed because of the slow tag.