Add tests/issues.rs, test Issue #1096 (branch 0.4.x)

[jtmoon79]

• Add new file tests/issues.rs for regression tests specific to issues.
• add test for Issue formatting NaiveDateTime as RFC3339 gives a bad-explaining error #1096

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If this PR is accepted, I will add another PR that moves all issue-specific tests into tests/issues.rs.

IMO, the issue-specific tests just make better sense in their own file. Since they cover niche topics it's often hard to know where to put issue-specific tests or what to title them. This code organization, placing them in tests/issues.rs, makes it easier to quickly add useful user-provided regression tests in a more organized manner.
User-provided issue-specific tests provide valuable certainty that a particular issue has been fixed.
They also exercise often niche behaviors that is too specific for "regular tests" to cover. Users spend a fair amount of time discovering then isolating an issue. Why not put that hard-won knowledge to use as another assurance of quality?
Issue-specific tests also demonstrate code-use for other users running into similar problems or with related questions.

I don't plan to add a test for all issues from the Issue backlog. But maybe going forward it can be made into a project practice that every fixed Issue gets a new test case in tests/issues.rs when that issue has reasonably simple reproduction steps provided,
e.g. a commit pattern like

1. add test to tests/issues.rs demonstrating failure (add test attribute #[should_panic] if the issue causes a panic)
2. code change with fix, adjust the test case for the fix

Issue-specific tests are not meant to replace "regular tests" or "general tests". Those should also be updated and augmented as necessary.

[djc]

As explained in #1091 (comment), I'm not a fan.