test: fix format of dictionaries

According to libFuzzer documentation [1] backslash should be escaped.

1. https://llvm.org/docs/LibFuzzer.html#dictionaries

```
$ swim_proto_meta_fuzzer -dict=swim_proto_meta_fuzzer.dict
ParseDictionaryFile: error in line 1
                "\001\000\000\004"
$ swim_proto_member_fuzzer -dict=swim_proto_member_fuzzer.dict
ParseDictionaryFile: error in line 1
                "\022\000\000\000\000\000\000\000"
```

NO_CHANGELOG=internal
NO_DOC=internal
NO_TEST=internal

(cherry picked from commit 62d03f15)
(cherry picked from commit 2b3eeda5)