RFR: 8301167: Update VerifySignedJar to actually exercise and test verification [v3]

[Eirik Bjorsnos]

> This PR resurrects VerifySignedJar which currently tests nothing.
> 
> VerifySignedJar currently verifies a binary JAR which was signed with SHA-1 back in April 2000. Because SHA-1 signed JARs has been disabled for a while, the JAR is treated as unsigned so the test doesn't really test anything as of now. 
> 
> The test is updated in the following ways:
> 
> - The JAR used for verification is now created and signed with SHA-256 by the test itself
> - The test is updated to check that the JAR is actually signed and with the expected certificate 
> - JarEntry InputStreams are now read fully to ensure verification of all entries
> - Objects.requireNonNull is used to check that entries returned by  getEntry, getJarEntry are non-null 
> - The existing binary JAR is retired

Eirik Bjorsnos has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge remote-tracking branch 'eirbjo/verify-signed-jar' into verify-signed-jar
 - Add whitespace after "if"
   
   Co-authored-by: Andrey Turbanov <turbanoff at gmail.com>
 - Remove unused local variables, replace Unreached/catch with runAndCheckException
 - Merge branch 'master' into verify-signed-jar
 - Simplify checkSignedBy by using JarEntry.getCertificates() instead of JarEntry.getCodeSigners()
 - o Update VerifySignedJar test to create and sign a JAR file instead of reading a binary JAR from version control.
   o Make sure that verification is actually triggered by reading the entire file contents of each entry
   o Check that entries are signed with the expected certificate.
   o Use Objects.requireNonNull to check that entries returned by  getEntry, getJarEntry are non-null
   o Retire the now unused binary test JAR "thawjar.jar"

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12206/files
  - new: https://git.openjdk.org/jdk/pull/12206/files/8ff06744..c26b27da

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12206&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12206&range=01-02

  Stats: 5306 lines in 266 files changed: 1548 ins; 1145 del; 2613 mod
  Patch: https://git.openjdk.org/jdk/pull/12206.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12206/head:pull/12206

PR: https://git.openjdk.org/jdk/pull/12206