Fix capture conversion in lenient mode.

[cpovirk]

...by always performing capture conversion in strict mode, where there
is a better-defined hierarchy (non-null < unspecified < nullable, with
no "but also unspecified < non-null if you want).

Fixes #193

[wmdietl]

Thanks for the detailed discussion of the fix in #193!
The fix makes sense. I'm wondering whether to add the test from #193 (comment) either as a regression test here or as a conformance test.

[cpovirk]

Good point. I'm adding a regression test for now to match what we've done before, but the regression directory should obviously be one source of possible conformance tests in the future. (There will be an interesting discussion to be had about which kinds of bugs justify per-checker tests vs. those which justify conformance tests. Maybe the right outcome will be that everything becomes a conformance test but that the borderline cases get thrown into a junk-drawer directory where no one expects organization? Maybe that wouldn't even be part of "the conformance tests" per se, just a part of the same repo for convenient integration? Or maybe that's all a bad idea.)

I've confirmed that the new test passes with JAVA_HOME=/usr/lib/jvm/java-17-openjdk-amd64 ./gradlew test --include-build ../jspecify --include-build ../checker-framework. (And I "intentionally" verified that it can fail by checking yet again whether I could remove the wonkyness workaround :) Yet again, I found that I can't.)