rust_test: Add a way to put items into runfiles without making them available at compile time

[abentley-ssimwave]

Currently, rust_test supports data, which is used at build time and also populates runfiles, and compile_data, which is used at build time but excluded from runfiles.

I want to be able to modify test input files without recompiling. These files are used exclusively at runtime, not build time, so changing them should not invalidate the test binary (but should invalidate the test results). I am currently using data, and this is causing unwanted compiles. What I want is the opposite of compile_data, e.g. runtime_data.

[dzbarsky]

See #3005