tests/typing: Replace hardcoded values with Hypothesis parameters
tests/typing: Replace hardcoded values with Hypothesis parameters

With hardcoded inputs, the testsuite would fail to catch some potential bugs,
as it only explores a single codepath of each method being tested:

  https://github.com/ppb/ppb-vector/pull/95#discussion_r244650431

I confirmed that the potential bug discussed in that message is now caught.

ppb/ppb-vector,ppb/ppb-vector