Disable dispatch_cascade due to hang in Swift CI

[seabaylea]

Temporarily disable the dispatch_cascade test as it hung in the Swift CI with the following log:

==================================================
[TEST] Dispatch Cascade
[PID] 19223
==================================================


[BEGIN] Process exited
    Actual: 1
    Expected: 0
[FAIL] Process exited (bsdtestharness.c:132)
    bsdtestharness.c:132
[PERF]  wall time: 0.034725
[PERF]  user time: 0.056000
[PERF]  system time: 0.008000
[PERF]  max resident set size: 17660
[PERF]  page faults: 0
[PERF]  swaps: 0
[PERF]  voluntary context switches: 43
[PERF]  involuntary context switches: 10
FAIL dispatch_cascade (exit status: 1)

Note that the process exited because the CI job killed it after 45 minutes.