Switch test-timer.c from s32 to s16 for better compatibility
Previously the test app used
SND_PCM_FORMAT_S32_LE which was unsupported on both of my USB audio interfaces. I switched to
SND_PCM_FORMAT_S16_LE which should hopefully have excellent compatibility (but probably has more dithering noise than strictly necessary, but it shouldn't matter for a timer test?). (I don't know if
SND_PCM_FORMAT_S24_LE is universally supported, but PipeWire uses that format for my FiiO E10K USB DAC. I didn't test on my cheap DAC.)
- I moved all format-specific types and values to a single block of typedefs and defines, acting as a single source of truth when changing the format.
- I used two spaces to separate the typedefs and defines, because I feel it's the easiest to read. Elsewhere in the file uses a single tab for this purpose, but that causes an inconsistent space between 1 and 8 spaces, so I chose to use 2 spaces instead.