Why are we able to measure the atmospheric pressure in the room and equate this to the pressure of a gas in a test tube?

Atmospheric pressure is measured with a barometer. A barometer consists of an evacuated vertical tube with its top end closed and its bottom end resting in a container of mercury that is open to the atmosphere The pressure exerted by the atmosphere acts on the exposed surface of the liquid to force mercury up into the tube. Sea level atmospheric pressure will support a mercury column generally not more than 29.92-in. high. Thus, the standard for atmospheric pressure at sea level is 29.92 in.-Hg, which translates to an absolute pressure of 14.69 psia.
The two basic reference points in all these measurements are standard atmospheric pressure and a perfect vacuum.