Flaky test failures for 0.8.7 on Rakudo 2022.04

[japhb]

First run (in fresh checkouts):

[...]
[Cro::HTTP] t/http-response.t ................. ok
[Cro::HTTP] t/http-reverse-proxy.t ............ 
[Cro::HTTP] All 19 subtests passed 
[Cro::HTTP] t/http-router-plugin.t ............ ok
[...]
[Cro::HTTP] Test Summary Report
[Cro::HTTP] -------------------
[Cro::HTTP] t/http-reverse-proxy.t          (Wstat: 0 Tests: 19 Failed: 0)
[Cro::HTTP]   Parse errors: Tests out of sequence.  Found (14) but expected (13)
[Cro::HTTP] Files=36, Tests=1714, 127 wallclock secs ( 0.29 usr  0.06 sys + 198.95 cusr 10.65 csys = 209.95 CPU)
[Cro::HTTP] Result: FAIL

Second attempt (again, in fully fresh checkouts):

[...]
[Cro::HTTP] t/http2-frame.t ................... ok
[Cro::HTTP] # Failed test 'Got a push promise when they are disabled!'
[Cro::HTTP] # at t/http2-push-promise.t line 61
[Cro::HTTP] # Failed test 'Got zero push promises when they are disabled'
[Cro::HTTP] # at t/http2-push-promise.t line 65
[Cro::HTTP] # expected: '0'
[Cro::HTTP] #      got: '1'
[Cro::HTTP] # You failed 2 tests of 7
[Cro::HTTP] t/http2-push-promise.t ............ 
[Cro::HTTP] Dubious, test returned 2 (wstat 512, 0x200)
[Cro::HTTP] Failed 2/7 subtests 
[Cro::HTTP] t/http2-request-parser.t .......... ok
[...]
[Cro::HTTP] Test Summary Report
[Cro::HTTP] -------------------
[Cro::HTTP] t/http2-push-promise.t          (Wstat: 512 Tests: 7 Failed: 2)
[Cro::HTTP]   Failed tests:  6-7
[Cro::HTTP]   Non-zero exit status: 2
[Cro::HTTP] Files=36, Tests=1715, 128 wallclock secs ( 0.27 usr  0.08 sys + 198.09 cusr 11.04 csys = 209.48 CPU)
[Cro::HTTP] Result: FAIL

Trying yet again, to see if there's yet another failure mode.

[japhb]

Third attempt worked. So three different outcomes in three runs. Sigh.