Fix create-obj / create-exe interaction

[fschutt]

Currently create-obj just creates a static_defs.h and a wasm.h file in the current directory whereas create-exe consumes this file. These tests are reliant on the current directory, so they break very easily if you run them from any other directory.

Currently this test will fail:

make build-wasmer
WASMER_DIR=./package 
    cargo llvm-cov --workspace --lcov 
    --output-path lcov.info 
    --features cranelift,singlepass,wasmer-artifact-create,static-artifact-create,wasmer-artifact-load,static-artifact-load,webc_runner 
    --exclude wasmer-integration-tests-ios

... because the static_defs.h is being generated, but not included in the create-exe. We need to find a way to make the interaction between static_defs.h and wasm.h as smooth as possible and also ideally not generate them in the current directory (since that might not be what the user wants). Also, we need to make tests more reliable by never assuming relative file paths, always canonicalize them and never rely on the current directory being stable (no ./blah.h)

For example, this file path needs to be fixed to be configurable:

wasmer/lib/cli/src/commands/create_exe.rs

Line 241 in 118b49e

&[std::path::Path::new("static_defs.h").into()],

... so that we can have an interaction between create-obj generating that static-defs.h file and create-exe consuming it.

[fschutt]

Tracking in #3393

[syrusakbary]

This was fixed by #3531