Tests should not "spam" system directories

[snazy]

Describe the bug

Some tests write test data to the system global /tmp:

/tmp/buckets
/tmp/ns
/tmp/ns1
/tmp/PolarisOverlappingTableTest
/tmp/test-private*.pem
/tmp/test-public*.pem

This is bad practice and should be replaced with JUnit's @TempDir. It can also easily cause issues when tests are run repeatedly.

[snazy]

This is a real issue causing false-positive test failures:

Caused by: org.apache.hadoop.fs.FileAlreadyExistsException: File already exists: file:/tmp/ns1/my_table/metadata/v1.metadata.json
	at org.apache.hadoop.fs.RawLocalFileSystem.create(RawLocalFileSystem.java:559)
	at org.apache.hadoop.fs.RawLocalFileSystem.create(RawLocalFileSystem.java:597)
	at org.apache.hadoop.fs.ChecksumFileSystem$ChecksumFSOutputSummer.<init>(ChecksumFileSystem.java:633)
	at org.apache.hadoop.fs.ChecksumFileSystem.create(ChecksumFileSystem.java:721)
	at org.apache.hadoop.fs.ChecksumFileSystem.create(ChecksumFileSystem.java:700)
	at org.apache.hadoop.fs.FileSystem.create(FileSystem.java:1233)
	at org.apache.hadoop.fs.FileSystem.create(FileSystem.java:1210)
	at org.apache.hadoop.fs.FileSystem.create(FileSystem.java:1091)