RFR: Fix @library tags in jtreg tests [v2]

[Maurizio Cimadamore]

> The `@library` tags in jtreg tests are wrong. According to the tag specification here:
> 
> https://openjdk.java.net/jtreg/tag-spec.html
> 
> If an absolute path is used in combination with `@library`, the path should be interpreted as either relative to the test root (e.g. test/), or relative to an external root path (specified with a property in TEST.ROOT). Since we're not using any property, it follows that the library path should be relative to `test/` - which means `/lib` is the correct choice here, not `/test/lib`.
> 
> (I discovered this when using the jtreg plugin [1] to run the jextract tests - these bad `@library` tags were causing a crash in the plugin; of course the plugin should be fixed - but the tags in the tests should be fixed as well).
> 
> [1] - https://github.com/openjdk/jtreg/tree/master/plugins/idea

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Drop unused test headers and unused properties in TEST.ROOT

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

Changes:
  - all: https://git.openjdk.java.net/jextract/pull/16/files
  - new: https://git.openjdk.java.net/jextract/pull/16/files/64a7d4e7..9d356297

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=16&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=16&range=00-01

  Stats: 227 lines in 31 files changed: 3 ins; 224 del; 0 mod
  Patch: https://git.openjdk.java.net/jextract/pull/16.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/16/head:pull/16

PR: https://git.openjdk.java.net/jextract/pull/16