Add test for relative extrusion resume kliment#1083

The test fails due to thread join unrelated to kliment#1083
This is fixed with next commit, then fails for missing M83 command,
which is fixed with another commit

testfiles/pause-restore-rel-e.gcode

@@ -0,0 +1,10 @@
+; test restoring of relative extrusion mode
+; after pause https://github.com/kliment/Printrun/issues/1083
+G28
+G90 ; abs all, including E
+M83 ; relative E
+;@!print('Please press Resume and check for M 83')
+;@!self.p.loud = True
+;@pause
+;@!self.p.loud = False
+;@!threading.Timer(2, lambda logbox: print('PASSED. Seen M 83' if 'M' + '83' in logbox.Value else 'FAILED: M 83 not seen'), (self.logbox,)).start()