[8u] RFR: 8227642: [TESTBUG] Make docker tests podman compatible

[Severin Gehwolf]

Hi!

Please review this test-only backport for OpenJDK 8u. I'd like to
backport this as it contains a test fix for TestCgroupMetrics
and TestSystemMetrics which occasionally can fail for equal values for
CPU usages (before/after). See changes in MetricsTester.java. On its
own it's useful too since it also makes the tests work with an
alternative container engine: podman.

Since this mostly changes test library code it's duplicated for jdk
tests in the jdk repo and hotspot tests in the hotspot repo. As such
the original JDK 11 patch doesn't apply cleanly. There is also no
jtreg-ext directory in 8u, so I've dropped the changes to VMProps.java.
It's not applicable. Some context changes in DockerTestUtils.java due
to JDK-8222888 and JDK-8222299, both not in 8u and no plan to backport
them at this point.

Bug: https://bugs.openjdk.java.net/browse/JDK-8227642
webrev: https://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8227642/jdk8/

Testing: Container tests on Linux x86_64 using docker/podman.

Thoughts?

Thanks,
Severin