Chart: fix endianness swap.

* This is most likely dead code currently (it is used only when the
framebuffer is big endian, and the the cpu is little endian)
* Fixes a warning.
* Can't confirm if B_SWAP_INT32 is really supposed to swap the value
in-place, or if this is a bug in Chart. Probably the latter.
Add more tests for BMessageFormat.

* Negative and null values, big values, thousands separators (in
different languages), string with no format, non-printable and non-utf8
charcaters, format not using # for the number.