How can I find which semi-professional camera has the resource to open the shutter for long exposure, with the purpose to take night shots?
All DSLRs have a Bulb mode which lets you open the shutter for a user-controlled time-interval. This is what you are looking for. Plenty of SLDs have it too.
There is a catch though! The amount time the shutter can be left open has a rarely documented limit on the vast majority of digital cameras. Sometimes it is in the order of a few minutes and sometimes in the order of a few hours. All Olympus cameras all stop after 30mins. Higher-end models can sustain an open shutter for hours as long as the battery holds and the sensor does not overheat. This later point will is non-deterministic because it has to do with the ambient temperature. With a Pentax K-7 for example, it I never managed more than about 30mins in the summer.
My advice is to narrow down your choice of cameras to a few you are interested in. At least in the class of a Canon 7D / Pentax K-5 / Nikon D7000 and then do a Google search for each model with bulb limit or long exposure. It may help for you to get a model which supports a battery-grip if you won't be able to use AC power where you are planning to shoot.