Tweeks to proptests

[Eh2406]

This is some small tweaks to the prop test configuration and adds an additional test.

Most of the diff is whitespace.

[rust-highfive]

r? @dwijnand

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[dwijnand]

Failure for Rust 1.31.0:

---- 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: [1099, 45, 31, 16, 20, 15, 3, 3, 0, 9]', tests/testsuite/support/resolver.rs:564:5

I'll assume it's spurious.

LGTM, enabling caching there looks like a good change.

@bors: r+

[bors]

📌 Commit d9e6360 has been approved by dwijnand

[bors]

⌛ Testing commit d9e6360 with merge 1b716f6...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: dwijnand
Pushing 1b716f6 to master...