exec_test: Extend tests to spec tests.
exec_test: Extend tests to spec tests.

This CL adds tests from a new folder testdata/spec, for running tests
from the WebAssembly specification's reference
interpreter (https://github.com/WebAssembly/spec/tree/62e01e2298d535a1773b4919b1f408c57b0b4e8e/test/core).
Tests can now parse hexadecimal numerical constants (integer and
floating point), and can test whether certain test cases panic or not.

go-interpreter/wagon,go-interpreter/wagon