perf test perftool_testsuite: Use absolute paths

perf test perftool_testsuite: Use absolute paths

Test cases from perftool_testsuite are affected by the current
directory where the test are run. For this reason, the test
driver has to change the directory to the base_dir for references to
work correctly.

Utilize absolute paths when sourcing and referencing other scripts so
that the current working directory doesn't impact the test cases.

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Jakub Brnak <jbrnak@redhat.com>
Signed-off-by: Michael Petlan <mpetlan@redhat.com>
Signed-off-by: Veronika Molnarova <vmolnaro@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>