RFR: Correct the build/test link to use the actual official links

[Magnus Ihse Bursie]

It was recently pointed out (in the discussion of JDK-8273497) that the Guide does not use the official link to build instructions, despite claiming so. Correct this. Also use official link for testing.

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Commit messages:
 - Correct the build/test link to use the actual official links

Changes: https://git.openjdk.java.net/guide/pull/64/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=64&range=00
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/guide/pull/64.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/64/head:pull/64

PR: https://git.openjdk.java.net/guide/pull/64