Add tests for SubgroupDispatch.

Add tests for SubgroupDispatch.

Introduced in v1.1, SubgroupDispatch adds the following:
- two new execution modes
- one new capability
- two new opcodes

Extend ValidateBase methods to take a spv_target_env.  Replace the
context_ member with ScopedContext inside the said methods.  Give
ScopedContext wider visibility by moving it outside
TextToBinaryTestBase.