RFR: Testing GHA [v3]

[Jorn Vernee]

> Add a basic actions workflow that runs the tests on pull requests

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  Change used LLVM binary to version specifically for Ubuntu 20.04

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Changes:
  - all: https://git.openjdk.java.net/jextract/pull/6/files
  - new: https://git.openjdk.java.net/jextract/pull/6/files/7cac234f..998e2adc

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=6&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=6&range=01-02

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jextract/pull/6.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/6/head:pull/6

PR: https://git.openjdk.java.net/jextract/pull/6