Test the corrupted .cmi bytes cases of read_cmi_string

A truncated value header, a truncated value and a malformed value each
report a corrupted interface. Codecov had these branches uncovered.

Signed-off-by: Jono Prest <jjprest@gmail.com>

Author: JonoPrest

tests/ounit_tests/ounit_compiler_request_tests.ml

@@ -1499,12 +1499,25 @@ let speculation_hook_tests _context =
       first_build := false;
       expect_code 0 (run_with_hooks root ~hooks "Dep.res");
       assert_equal new_bytes (file_contents cmi_path);
+      let corrupted bytes =
+        try
+          ignore (Cmi_format.read_cmi_string "Dep.cmi" bytes);
+          false
+        with Cmi_format.Error (Corrupted_interface _) -> true
+      in
+      let magic_length = String.length Config.cmi_magic_number in
+      check (corrupted "Caml") "truncated bytes are a corrupted interface";
+      check
+        (corrupted (String.sub new_bytes 0 (magic_length + 1)))
+        "a truncated value header is a corrupted interface";
+      check
+        (corrupted
+           (String.sub new_bytes 0 (magic_length + Marshal.header_size + 1)))
+        "a truncated value is a corrupted interface";
       check
-        (try
-           ignore (Cmi_format.read_cmi_string "Dep.cmi" "Caml");
-           false
-         with Cmi_format.Error _ -> true)
-        "truncated bytes are a corrupted interface";
+        (corrupted
+           (Config.cmi_magic_number ^ String.make Marshal.header_size 'x'))
+        "a malformed value is a corrupted interface";
       (* The published file is older than the redirected bytes. *)
       write root "Dep.res" "let value = 1\n";
       expect_code 0 (run root "Dep.res" |> snd);