rtl_test: Replace the R820T / E4K tuner-specific tests with a generic tuner range test.

This scans 0 - 3GHz looking for frequencies that can be tuned to without error, and
refines the band edges further once a tuneable band is found. It should work with any tuner
that correctly reports tuning errors. It takes about 1.5 minutes to complete on a R820T.