Consider removing E2E_GROUPS / E2E_TEST for isolated e2e test runs

[dtrudg]

Type of issue

technical debt

Description of issue

In the 3.11 cycle, we added E2E_GROUPS and E2E_TEST options to the e2e-tests make target to filter the tests that are run.

This was initially put in place because using scripts/e2e-test with -run was complicated by the two modes of e2e execution (with / without PID namespace) introduced when systemd cgroups management was added.

Additionally, it was felt that since make -C builddir e2e-test is the entry point to run the test suite, that it was friendly to put something there.

We did eventually implement process reaping, following apptainer... removing the PID namespace for all tests, so we don't actually have the two modes of e2e test execution now.

Perhaps it's better to promote running individual e2e tests with scripts/e2e-test -run xxxx instead? Needs explanation about the TestE2E/PAR and TestE2E/Seq prefix... etc.

Or is it that more confusing for people not familiar with go test... and we should leave the makefile driven filters?