RunTest invokes a data-driven test.

RunTestFromString is a version of RunTest which takes the contents of a test directly.

Walk goes through all the files in a subdirectory, creating subtests to match the file hierarchy; for each "leaf" file, the given function is called.

CmdArg contains information about an argument on the directive line.

TestData contains information about one data-driven test case that was parsed from the test file.