Adding the start of the instruction test infrastructure.

test/codegen/README.md

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+# Codegen Tests
+
+This directory contains the test assets used by the automated codegen test
+runner.
+
+Each test is structured as a source `[name].s` PPC assembly file and the
+generated outputs. The outputs are made using the custom build of binutils
+setup when `xenia-build setup` is called and are checked in to make it easier
+to run the tests on Windows.
+
+Tests are run using the `xenia-test` app or via `xenia-build test`.
+
+## Execution
+
+The test binary is placed into memory at `0x82010000` and all other memory is
+zeroed.
+
+All registers are reset to zero. In order to provide useful inputs tests can
+specify `# REGISTER_IN` values.
+
+The code is jumped into at the starting address and executed until the last
+instruction in the input file is reached.
+
+After all instructions complete any `# REGISTER_OUT` values are checked and if
+they do not match the test is failed.
+
+## Annotations
+
+Annotations can appear at any line in a file. If a number is required it can
+be in either hex or decimal form, or IEEE if floating-point.
+
+### REGISTER_IN
+
+```
+# REGISTER_IN [register name] [register value]
+```
+
+Sets the value of a register prior to executing the instructions.
+
+Examples:
+```
+# REGISTER_IN r4 0x1234
+# REGISTER_IN r4 5678
+```
+
+### REGISTER_OUT
+
+```
+# REGISTER_OUT [register name] [register value]
+```
+
+Defines the expected register value when the instructions have executed.
+If after all instructions have completed the register value does not match
+the value given here the test will fail.
+
+Examples:
+```
+# REGISTER_OUT r3 123
+```
+
+TODO: memory setup/assertions