trial: always use abspath

No actual failure known, but using the abspath will prevent further problems
when running a binary in its own run dir and passing trial paths to it.

Adjust test expectations.

Change-Id: Ia2ba876a42eef9122a92645edd1f28ddb56e6be3
diff --git a/selftest/trial_test.ok b/selftest/trial_test.ok
index 66dcff0..6ad39a9 100644
--- a/selftest/trial_test.ok
+++ b/selftest/trial_test.ok
@@ -11,6 +11,6 @@
 None
 - test checksum verification
 - detect wrong checksum
-ok, got RuntimeError("Checksum mismatch for 'trial_test/invalid_checksum/file2' vs. 'trial_test/invalid_checksum/checksums.md5' line 2",)
+ok, got RuntimeError("Checksum mismatch for '[PATH]/trial_test/invalid_checksum/file2' vs. '[PATH]/trial_test/invalid_checksum/checksums.md5' line 2",)
 - detect missing file
-ok, got RuntimeError("File listed in checksums file but missing in trials dir: 'trial_test/missing_file/file2' vs. 'trial_test/missing_file/checksums.md5' line 2",)
+ok, got RuntimeError("File listed in checksums file but missing in trials dir: '[PATH]/trial_test/missing_file/file2' vs. '[PATH]/trial_test/missing_file/checksums.md5' line 2",)