Fix build failures when cmake never used

FISH_BUILD_DIR (nominally, ./build) is created by cmake. If you only check out
the project via git and then run `cargo build`, this directory won't exist and
many of the tests will fail.

src/tests/mod.rs

@@ -65,6 +65,9 @@ pub fn popd() {
     pub fn test_init() -> impl ScopeGuarding<Target = ()> {
         static DONE: OnceCell<()> = OnceCell::new();
         DONE.get_or_init(|| {
+            // If we are building with `cargo build` and have build w/ `cmake`, FISH_BUILD_DIR might
+            // not yet exist.
+            std::fs::create_dir_all(env!("FISH_BUILD_DIR")).unwrap();
             set_current_dir(env!("FISH_BUILD_DIR")).unwrap();
             {
                 let s = CString::new("").unwrap();