RFR: 8303920: Avoid calling out to python in DataDescriptorSignatureMissing test [v9]

[Eirik Bjorsnos]

> Please review this PR which brings  the DataDescriptorSignatureMissing test back to life.
> 
> This test currently calls out to Python to create a test vector ZIP with a Data Descriptor without the recommended but optional signature. The Python dependency has turned out to be very brittle, so the test is currently marked with `@ignore` 
> 
> The PR replaces Python callouts with directly creating the test vector ZIP in the test itself. We can then remove the `@ignore`tag and run this useful test automatically.

Eirik Bjorsnos has updated the pull request incrementally with three additional commits since the last revision:

 - Describe the structure of Data Descriptors using an example
 - Extend the comment of `makeZipWithSignaturelessDescriptor` with some background info on (signature-less) data descriptors, for the benefit of future maintainers.
 - Convert test from testng to junit

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12959/files
  - new: https://git.openjdk.org/jdk/pull/12959/files/de80380f..4a541ffb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12959&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12959&range=07-08

  Stats: 43 lines in 1 file changed: 34 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/12959.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/12959/head:pull/12959

PR: https://git.openjdk.org/jdk/pull/12959