Add tests for Myanmar Asat+MedialYa and MedialYa+Asat sequences

One of them currently produces dotted-circle.  Fix and detailed
message coming.
[myanmar] Allow MedialYa+Asat in the grammar

The grammar in the OT spec, and the existing Windows implementation
seem to be confused around where to allow Asat around the medial
consonants.

The previous grammar for medial group was allowing an Asat after
the medial group only if there was a medial Wa or Ha, but not if
there was only a medial Ya.  This doesn't make sense to me and
sounds reversed, as both medial Wa and Ha are below marks while
Asat is an above mark.  An Asat can come before the medial group
already (in fact, multiple ones can.  Why?!).  The medial Ya
however is a spacing mark and according to Roozbeh it's valid
to want an Asat on the medial Ya instead of the base, so it looks
to me like we want to allow an Asat after the medial group if
there *was* a Ya but not if there wasn't any.  Not wanting to
produce dotted-circle where Windows is not, this commit changes
the grammar to allow one Asat after the medial group no matter
what comes in the group.

Test: U+1002,103A,103B vs U+1002,103B,103A
Fix build with --coretext on older OS X

Fixes https://github.com/behdad/harfbuzz/pull/40