tests: drivers: build_all: optimize built drivers

All generic drivers, ie, emulators/SPI/I2C based devices, were defined
in app.overlay, meaning the overlay is picked by all builds. However,
what was defined in app.overlay was only used by the native_posix build.
This will save some CI time, as generic drivers were being built more
than strictly necessary.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>