Better testing for UnicodeSets

[skius]

UnicodeSets should have more positive tests.

If conformance tests are create for UnicodeSets, we should add those too.

For now, ICU4C/ICU4J can be used as inspiration:

• https://github.com/unicode-org/icu/blob/main/icu4j/main/tests/core/src/com/ibm/icu/dev/test/lang/UnicodeSetTest.java
• https://github.com/unicode-org/icu/blob/main/icu4c/source/test/intltest/usettest.cpp

[skius]

More tests have been added since that PR, in particular in the spec-draft-compliance-PR #3670. They cover a good range of use cases.

What's still left is to decide whether we want conformance tests from upstream (which do not currently exist) before stabilizing this.