testsuite: use relative paths, set PATH to absolute path

This approach works better with autotools, which supports the build
directory being complete outside the source tree.

testcases/t/201-config-parser.t

@@ -25,7 +25,7 @@ sub parser_calls {
     my ($command) = @_;
 
     my $stdout;
-    run [ '../test.config_parser', $command ],
+    run [ 'test.config_parser', $command ],
         '>/dev/null',
         '2>', \$stdout;
     # TODO: use a timeout, so that we can error out if it doesn’t terminate