test: Robustify TestGrafanaClient for host name changes
test: Robustify TestGrafanaClient for host name changes

Avoid dynamic host name changes during PCP data collection. Since [1]
these now happen on a few images. Instead, set a non-transient fixed
host name for the guest and start PCP log collection from a clean slate,
similar to the other metrics tests.

This avoids a RHEL/CentOS special case and pins down the expected
strings.

[1] https://github.com/cockpit-project/bots/commit/9450dad0b0f5

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