mlir/tblgen test: add a test for EnumAttr customAssemblyFormat

attr-or-type-format.td contains tests for various attributes and types,
but nowhere in the testsuite is the customAssemblyFormat for an EnumAttr
(enum class in C++) exercised. Fix this by adding a test.

Signed-off-by: Ramkumar Ramachandra <r@artagnon.com>

Differential Revision: https://reviews.llvm.org/D140034