RFR: Update to Oracle Linux 7.6

[Erik Helin]

Hi all,

this small patch updates the base image in `test.dockerfile` from Oracle Linux 7.5 to 7.6.

## Testing
- [x] `sh gradlew reproduce`

Thanks,
Erik

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

Commits:
 - 1c931299:	Update to Oracle Linux 7.6

Pull request:
https://git.openjdk.java.net/skara/pull/141

Webrev:
https://webrevs.openjdk.java.net/skara/141/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/141.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/141/head:pull/141