[mlir][test] Reorganize the test dialect (#89424)

This PR massively reorganizes the Test dialect's source files. It moves
manually-written op hooks into `TestOpDefs.cpp`, moves format custom
directive parsers and printers into `TestFormatUtils`, adds missing
comment blocks, and moves around where generated source files are
included for types, attributes, enums, etc. into their own source file.

This will hopefully help navigate the test dialect source code, but also
speeds up compile time of the test dialect by putting generated source
files into separate compilation units.

This also sets up the test dialect to shard its op definitions, done in
the next PR.