Figure out what do do about tests that fail on i386 only
We have had to disable some tests because they render differently on i386 (32-bit machines):
There are differences in rounding when rendering paths, and differences in text rendering.
- Have a list of tests to exclude on i386 only, rather than ignoring them everywhere.
- Have different test reference files for i386. However, they render differently again depending on whether librsvg is built in debug mode or in release mode.