transpile: tests: rename main.c to main_fn.c since the main.rs generated is detected as a test (#1499)

When testing `main.c`, `main.rs` is generated, and `cargo` thinks
`c2rust-transpile/tests/snapshots/main.rs` is a second `snapshots` test,
in addition to `c2rust-transpile/tests/snapshots.rs`:

```shell
❯ cargo test -p c2rust-transpile
error: failed to load manifest for workspace member `/home/kkysen/work/c2rust/c2rust`

Caused by:
  failed to load manifest for dependency `c2rust-transpile`

Caused by:
  failed to parse manifest at `/home/kkysen/work/c2rust/c2rust-transpile/Cargo.toml`

Caused by:
  found duplicate test name snapshots, but all test targets must have a unique name
```

This was from #1447, but wasn't caught in CI because it only runs `cargo
test` a single time.

Author: kkysen

c2rust-transpile/tests/snapshots/main.c …rust-transpile/tests/snapshots/main_fn.cc2rust-transpile/tests/snapshots/main.c renamed to c2rust-transpile/tests/snapshots/main_fn.c c2rust-transpile/tests/snapshots/main.c renamed to c2rust-transpile/tests/snapshots/main_fn.c

@@ -1,7 +1,7 @@
 ---
 source: c2rust-transpile/tests/snapshots.rs
-expression: cat tests/snapshots/main.rs
-input_file: c2rust-transpile/tests/snapshots/main.c
+expression: cat tests/snapshots/main_fn.rs
+input_file: c2rust-transpile/tests/snapshots/main_fn.c
 ---
 #![allow(
     dead_code,