This should make the unit tests more readable, and it also prevents
failures where a changed fromXML() implementation fails to ignore
interspersed whitespace. This has recently caused some developer
nuisance; see https://github.com/behdad/fonttools/pull/367.
Instead of having a custom parser implementation, it would be nicer
to call the actual XMLReader. However, XMLReader has a deeply built-in
assumption that it is processing an entire TTX file. Changing this
assumption would certainly be possible, but it would need a re-write
of the XMLReader code; having a custom parser just for unit tests
seems less involved.
Resolves https://github.com/behdad/fonttools/issues/355
For making sure that `pyftsubset` still works after this change,
I have done the following steps:
* invoked Adobe's `makeotf` tool to build a custom font with a
MultipleSubst lookup. This lookup decomposes two two ligatures,
`c_t` and `f_f_i`, into their respective components.
* invoked the `pyftsubset` tool to produce a subset font with just
the `c_t` ligature;
* checked with `ttx` that the newly produced subset font contains
the requested `c_t` ligature and its components `c` and `t`,
but does not contain not any of `f_f_i`, `f`, or `i`.
