OpSpecConstantOp contains an embedded opcode which is given as a literal
argument to the OpSpecConstantOp. The subsequent arguments are as the
embedded op would expect, which may be a mixture of IDs and literals. This
adds support for that to the remapper binary parser. Upon seeing such an
embedded op, the parser flips over to parsing the argument list as
appropriate for that opcode.
Fixes#882.
The gtest executable accepts a --test-root option to specify
a root directory for test files. It defaults to the Test directory
in the source tree from which the executable is built.
For example, this lets us run test exectuables built with MinGW on Linux
on a Windows machine with its own copy of the source tree.
