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

The new benchmark code 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.

This can be pretty noisy against the existing code, as the r820t tuner reports tuning errors to stderr and the search process generates a lot of tuning errors. Suppressing the error reports yields this, which is interesting in itself - apparently the PLL will claim to lock at all sorts of odd frequencies:

> $ ./rtl_test -t
> [...]
> Testing tuner range. This may take a couple of minutes..
> Found a new band starting at 3450043 Hz
> Tuning band: 3450043 - 7828464 Hz
> Found a new band starting at 12900050 Hz
> Tuning band: 12900050 - 1851682318 Hz
> Found a new band starting at 1851682379 Hz
> Tuning band: 1851682379 - 1851694035 Hz
> Found a new band starting at 1851694097 Hz
> Tuning band: 1851694097 - 1851694341 Hz
> Found a new band starting at 1851694402 Hz
> Tuning band: 1851694402 - 1851694402 Hz