transpile: snapshots: add dummy files so insta::glob sees multiple tests

insta handily uses different snapshot file naming schemes dependent on whether a glob finds one or multiple files. as we only have one platform-dependent test yet, it doesn't include the @filename suffix, but it will when we add more.

for now, add a dummy test so that insta always uses the `@file.c` suffix; we should delete the dummy test when we have more platform-specific tests.

Author: fw-immunant

c2rust-transpile/tests/snapshots/platform-specific/dummy.c

@@ -0,0 +1,2 @@
+/* empty file until we have multiple tests; if we glob only one test, insta
+omits the `@file.c` suffix from snapshots */

c2rust-transpile/tests/snapshots/platform-specific/dummy.rs

@@ -0,0 +1,9 @@
+#![allow(
+    dead_code,
+    mutable_transmutes,
+    non_camel_case_types,
+    non_snake_case,
+    non_upper_case_globals,
+    unused_assignments,
+    unused_mut
+)]

c2rust-transpile/tests/snapshots/[email protected]

@@ -0,0 +1,16 @@
+---
+source: c2rust-transpile/tests/snapshots.rs
+assertion_line: 69
+expression: cat tests/snapshots/platform-specific/dummy.rs
+input_file: c2rust-transpile/tests/snapshots/platform-specific/dummy.c
+---
+#![allow(
+    dead_code,
+    mutable_transmutes,
+    non_camel_case_types,
+    non_snake_case,
+    non_upper_case_globals,
+    unused_assignments,
+    unused_mut
+)]
+

c2rust-transpile/tests/snapshots/[email protected]

@@ -0,0 +1,16 @@
+---
+source: c2rust-transpile/tests/snapshots.rs
+assertion_line: 69
+expression: cat tests/snapshots/platform-specific/dummy.rs
+input_file: c2rust-transpile/tests/snapshots/platform-specific/dummy.c
+---
+#![allow(
+    dead_code,
+    mutable_transmutes,
+    non_camel_case_types,
+    non_snake_case,
+    non_upper_case_globals,
+    unused_assignments,
+    unused_mut
+)]
+