transpile: tests: stop using insta::glob! for c_decls_snapshots, too

Since there's only a single `c_decls_snapshots`,
I didn't add any extra code like `TranspileTest` for the transpile snapshots.

Also, previously, the code wasn't accounting for needing a `dummy`
and used the absolute path in the debug expression, which changes.
Now both are fixed and a relative path is used.

Author: kkysen

c2rust-transpile/tests/snapshots.rs

@@ -419,11 +419,6 @@ fn test_varargs() {
         .run();
 }
 
-#[test]
-fn check_c_decl_map() {
-    insta::glob!("c_decls_snapshots/*.c", transpile_with_c_decl_map_snapshot);
-}
-
 fn transpile_with_c_decl_map_snapshot(c_path: &Path) {
     compile_and_transpile_file(c_path, config());
 
@@ -434,3 +429,9 @@ fn transpile_with_c_decl_map_snapshot(c_path: &Path) {
         serde_json::from_str(&fs::read_to_string(&c_decls_path).unwrap()).unwrap();
     insta::assert_json_snapshot!(snapshot_name, json, &debug_expr);
 }
+
+#[test]
+fn test_nh() {
+    let c_path = Path::new("tests/c_decls_snapshots/nh.c");
+    transpile_with_c_decl_map_snapshot(c_path);
+}

c2rust-transpile/tests/snapshots/snapshots__c_decls-nh.c.snap

@@ -1,7 +1,6 @@
 ---
 source: c2rust-transpile/tests/snapshots.rs
-expression: cat /home/mod/immunant/c2rust/repo/c2rust-transpile/tests/c_decls_snapshots/nh.c_decls.json
-input_file: c2rust-transpile/tests/c_decls_snapshots/nh.c
+expression: cat tests/c_decls_snapshots/nh.c_decls.json
 ---
 {
   "FOO": "FOO 4000+50",