Improve readability of the test JarWithOneNonDisabledDigestAlg

This PR attempts to make JarWithOneNonDisabledDigestAlg a little easier to read.

- Some changes are made in the choice of algorithms and naming. The intent here is to reduce confusion and make the purpose of the test clearer:
- Updated the enabled digestAlgorithm in use from SHA1 to SHA256. The use of SHA1 here seems just a bit confusing, since it has been considered weak for a while
- The two different signer aliases are now named SIGNER1, SIGNER2 instead of the somewhat confusing SHA1, SHA256
- Both signing keys are now generated with -sigalg SHA256withRSA since the sigalg of the keys does not seem to matter for this test

There are also some general code cleanups:

- Moved loading of the key store into the new method loadKeyStore
- Updated checkThatJarIsSigned to take a parameter Map<String, Integer> representing the expected signer counts for each path in the JAR. This provides a cleaner separation between expectiations and the enforcement of expectations.
- Introduced Path constants for various file names used throughout the test, reducing a number of redundant Path.of calls which seemed to clutter the code a bit
Updated IO code to use new APIs, such as Files.newOutputStream, Files.newInputStream, InputStream.transferTo and OutputStream.nullOutputStream.
 -Added/updated some comments where appropriate