Migrate remaining test to JUnit 5

[timo-a]

Since this is going to take several PRs, this issue is intended to coordinate them.

original PR: #1237

• Part 1: update(tests): migrate JUnit 4 code to JUnit 5 in independent tests #1238 tests that don't extend or depend on BaseTest or its child classes
• Part 2: update(tests): migrate JUnit 4 code to JUnit 5 in async tests #1244 tests that extend AsyncTestBase
• Part 3: update(tests): migrate JUnit 4 code to JUnit 5 in core.read #1245 tests in core.read that extend BaseTest
• Part 4 update(tests): migrate JUnit 4 code to JUnit 5 in core.io, core.json and failing #1246 core.io, core.json and failing
• Part 5 update(tests): migrate JUnit 4 code to JUnit 5 in core.base64 to core.sym #1247 core.base64 to core.sym
• Part 6 update(tests): migrate remaining JUnit 4 code in core to JUnit 5 #1248 remaining tests

[cowtowncoder]

First PR merged; looks like there are 125 cases of BaseTest being extended so likely target count.

May need 3 - 5 PRs depending on what works best. Since 2.18 branch only now starting, ok to target against 2.17.

[timo-a]

I guess we're done here, junit-vintage-engine is gone from the pom files of master, 2.17 and 2.28.

[cowtowncoder]

Indeed! Thank you for doing this, @timo-a !