tests: Increase timeout in test_changing_master_during_build

This test was often failing because of a timeout caused by lost
MATOCL_FUSE_WRITE_CHUNK messages which results in locked chunks that
block clients for 120 seconds each time the situation occurs. This
commit fixes the issue by increasing the timeout for this test to
40 minutes.

Change-Id: I81d21994683eeaeabed86b8d5113db33298e3b4a