RFR: 8300272: Improve readability of the test JarWithOneNonDisabledDigestAlg [v4]

[Eirik Bjorsnos]

> 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

Eirik Bjorsnos has updated the pull request incrementally with one additional commit since the last revision:

  Add whitespace between if and left parenthesis

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11997/files
  - new: https://git.openjdk.org/jdk/pull/11997/files/11649bc9..8081b013

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11997&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11997&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11997.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11997/head:pull/11997

PR: https://git.openjdk.org/jdk/pull/11997