meta_test_multiple_versions_strategy is spuriously failing

[ehuss]

meta_test_multiple_versions_strategy has failed a few times during CI:

• Nov 9: Do not attempt to ascribe projections out of a ty var rust#55637 (comment)
• Jan 25: Tweeks to proptests #6599 (comment)
• Jan 29: Support multiple versions of MdBook for docs rust#57495 (comment)

with errors that look like:

thread 'support::resolver::meta_test_multiple_versions_strategy' panicked at 'In 1280 tries we did not see a wide enough distribution of multiple versions of the same library! dis: [1099, 45, 31, 16, 20, 15, 3, 3, 0, 9]', tests/testsuite/support/resolver.rs:564:5

thread 'support::resolver::meta_test_multiple_versions_strategy' panicked at 'In 1280 tries we did not see a wide enough distribution of multiple versions of the same library! dis: [1026, 75, 30, 24, 14, 5, 4, 7, 0, 19]', src/tools/cargo/tests/testsuite/support/resolver.rs:564:5

[ehuss]

Hit again today.
---- support::resolver::meta_test_multiple_versions_strategy stdout ----
thread 'support::resolver::meta_test_multiple_versions_strategy' panicked at 'In 1280 tries we did not see a wide enough distribution of multiple versions of the same library! dis: [1086, 55, 16, 11, 9, 10, 3, 10, 0, 22]', tests/testsuite/support/resolver.rs:564:5

[Eh2406]

So with the number of different things running Cargos test sweet flaky tests are not acceptable. I am sorry for adding them! It does make me wonder if something changed to make the distribution worse. The real solution is to switch to using a fixed seed so as to be sure we have consistent tests.

[ehuss]

Also occurs with meta_test_deep_trees_from_strategy
https://travis-ci.com/rust-lang/cargo/jobs/178165928

thread 'support::resolver::meta_test_deep_trees_from_strategy' panicked at 'In 1280 tries we did not see a wide enough distribution of dependency trees! dis: [49, 852, 94, 43, 32, 23, 21, 24, 16, 16, 17, 10, 9, 7, 5, 9, 8, 4, 4, 0, 23]', tests/testsuite/support/resolver.rs:524:5

[mati865]

Fixed by #6753?

[Eh2406]

Yes! Thanks!