tests: don't log filename nor color in unit tests

In experr, we had line numbers in log output, which might change. Also, for
make distcheck, the path of the source file might have some '../../' added in
the log output (to indicate the src dir as seen from the build dir).

Fix both by dropping source file and line.

Also drop color while at it.

Change-Id: Ie76384c4176ce0a7d89d093f2efb848fe3f19400

src/tests/test-helpers.err

@@ -1,3 +1,2 @@
-<0004> ranap_common.c:508 Invalid PLMN Identity size: should be 3, is 2
-<0004> ranap_common.c:522 Invalid LAC size: should be 2, is 1
-
+Invalid PLMN Identity size: should be 3, is 2
+Invalid LAC size: should be 2, is 1

src/tests/test_common.c

@@ -85,4 +85,7 @@ int test_common_init(void)
 		exit(1);
 
 	ranap_set_log_area(DRANAP);
+
+	log_set_print_filename(osmo_stderr_target, 0);
+	log_set_use_color(osmo_stderr_target, 0);
 }