[mlir] somewhat decompose TestDialect.cpp

TestDialect.cpp along with the ODS-generated files amounts to around
100k LoC and takes a significant amount of time to compile. Factor out
the test ops related to testing the sytnax and assembly format, which
are a relatively large and well delimited group, into a separate set of
files.

Also factor out dialect interfaces into a separate file.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D155947