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

selftest/trial_test.ok

@@ -11,6 +11,6 @@ third
 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",)

selftest/trial_test.ok.ign

@@ -1,2 +1,3 @@
 /tmp/[^/]*	[TMP]
 ....-..-.._..-..-..	[TIMESTAMP]
+'[^']*/trial_test	'[PATH]/trial_test

src/osmo_gsm_tester/trial.py

@@ -52,7 +52,7 @@ class Trial(log.Origin):
             return Trial(trials_dir.child(next_trial)).take()
 
     def __init__(self, trial_dir):
-        self.path = trial_dir
+        self.path = os.path.abspath(trial_dir)
         self.set_name(os.path.basename(self.path))
         self.set_log_category(log.C_TST)
         self.dir = util.Dir(self.path)