Merge remote-tracking branch 'remotes/philmd-gitlab/tags/acceptance-fixes-20200407...

Merge remote-tracking branch 'remotes/philmd-gitlab/tags/acceptance-fixes-20200407' into staging

Acceptance tests patches

- Fixed EXEC migration
- Reduce PReP/40p artifacts download failures,
- Disable Leon3 HelenOS
- Speed up Travis-CI job by using a specific cache bucket

CI jobs results:
  https://gitlab.com/philmd/qemu/pipelines/133379305
  https://travis-ci.org/github/philmd/qemu/builds/671762970

# gpg: Signature made Tue 07 Apr 2020 11:37:57 BST
# gpg:                using RSA key FAABE75E12917221DCFD6BB2E3E32C2CDEADC0DE
# gpg: Good signature from "Philippe Mathieu-Daudé (F4BUG) <f4bug@amsat.org>" [full]
# Primary key fingerprint: FAAB E75E 1291 7221 DCFD  6BB2 E3E3 2C2C DEAD C0DE

* remotes/philmd-gitlab/tags/acceptance-fixes-20200407:
  .travis.yml: Cache acceptance-test assets
  tests/acceptance/machine_sparc_leon3: Disable HelenOS test
  tests/acceptance/ppc_prep_40p: Use mirror for ftp.software.ibm.com
  tests/acceptance/ppc_prep_40p: Use cdn.netbsd.org hostname
  Acceptance test: Fix to EXEC migration

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>