Move einsum's test data to constexpr variables (#19320)

### Description
emscripten's C++ compiler has difficulty on compiling einsum_test.cc
because the file has too many local variables. So I moved them to
constexpr.

Author: snnn

cmake/onnxruntime_unittests.cmake

@@ -825,8 +825,7 @@ if (CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
     )
   endif()
   list(REMOVE_ITEM all_tests "${TEST_SRC_DIR}/providers/cpu/reduction/reduction_ops_test.cc"
-      "${TEST_SRC_DIR}/providers/cpu/tensor/grid_sample_test.cc"
-      "${TEST_SRC_DIR}/providers/cpu/math/einsum_test.cc")
+      "${TEST_SRC_DIR}/providers/cpu/tensor/grid_sample_test.cc")
 endif()
 
 set(test_all_args)