RFR: Fix @library tags in jtreg tests

[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

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Commit messages:
 - Fix @library tags in tests

Changes: https://git.openjdk.java.net/jextract/pull/16/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=16&range=00
  Stats: 26 lines in 26 files changed: 0 ins; 0 del; 26 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