Migrated JVM tests from JUnit 4 to JUnit 5

[wilburx9]

Basically, the title but here's a summary of the changes:

• I basically switched the JUnit 4 dependency in the okio and samples modules to JUnit 5 and subsequently updated the imports.
• I have tried to do this with as little changes as possible, but the tests in FakeFileSystemTest require updates to their constructors since JUnit 5 doesn't support multiple constructors.
• I also added a helper function to create a temporary file after migrating from JUnit 4's TemporaryFolder to JUnit 5's @TempDir

This is the first PR for implementing #1846

[wilburx9]

okio.WasiFileSystemTest.fileMetadataTimestampsAreDistinct is failing on my local machine, both in the main branch and this branch. Hopefully it's only a local issue; happy to investigate otherwise.

[swankjesse]

CI has this failure only:

OkioKotlinTest[jvm] > fileSink()[jvm] FAILED
    org.junit.platform.commons.JUnitException: Failed to close extension context
        at java.base@26.0.2/java.util.ArrayList.forEach(ArrayList.java:1612)
        at java.base@26.0.2/java.util.ArrayList.forEach(ArrayList.java:1612)

        Caused by:
        java.io.IOException: Failed to delete temp directory C:\Users\RUNNER~1\AppData\Local\Temp\junit-2733229505807075268. The following paths could not be deleted (see suppressed exceptions for details): <root>, tmp_file2143762405183197976.tmp
            at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:186)
            at java.base/java.util.ArrayList.forEach(ArrayList.java:1612)
            at java.base/java.util.stream.SortedOps$RefSortingSink.end(SortedOps.java:395)
            at java.base/java.util.stream.Sink$ChainedReference.end(Sink.java:261)
            at java.base/java.util.stream.Sink$ChainedReference.end(Sink.java:261)
            at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:571)
            at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:560)
            at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:153)
            at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:176)
            at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:265)
            at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:632)
            ... 2 more

[wilburx9]

@swankjesse Should be fixed now