Revive SkSLFPTestbed for basic verification of CPP/H codegen.

This brings back the basics from SkSLFPTest.cpp. This file was removed
entirely in http://review.skia.org/319029 but, in retrospect, it's still
a good idea for dm to verify that CPPCodeGen and HCodeGen can do their
jobs. And, like SkSLGLSLTestbed, this gives us a good place to attach
the debugger in dm for testing CPP/H-specific code generation bugs.

Change-Id: I514192bacd63021708dbd02a0276a3d55a43195f
Bug: skia:10684
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/329370
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed