Refactor end to end test

[zugzwang]

Refactor end-to-end tests

TestEndToEnd acts like the functional tests of the openpgp package: Take any key (generated on-the-fly or external), encrypt/sign messages to other keys, and decrypt/verify.

Currently, every end-to-end test is happening in end_to_end_test.go, inside the openpgp package. This needs some refactoring:

• It does not give a clear idea of how the openpgp package API is used.
• The file is too long and not easy to read.
• There are some exported functions.

Separate integrationtests package

Create a separate package that imports openpgp and gives a clear idea on how to use the API, while improving readability. Future integration tests with other libraries (such as OpenPGPjs) should be included here.

Split file

• end_to_end_test.go contains TestEndToEnd and other helper functions that involve objects from the testing package.
• utils.go contains helper functions (mostly format functions).
• test_vectors.go contains test vectors such as external keys and string constants.

JSON test vectors instead of global variables

end_to_end_test.go contains large strings as global variables. It is better to have this data as a separate JSON. Also, this serves the purpose of exporting/importing test vectors easier fro cross-library testing.
The only caveat is that encoding/json can only handle structs with exported fields, therefore, all relevant keys in JSON maps must start with a capital letter.

[twiss]

Looks good to me, other than nits below 👍