Relative/Absolute path improvement for generate.sh

[flavioheleno]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	none
Documentation	if this is a new feature, link to pull request in https://github.com/php-http/documentation that adds relevant documentation
License	MIT

What's in this PR?

This PR does 3 simple changes:

1. It removes the need of changing the current directory to tests/server/ssl before executing generate.sh;
2. It adds the flags below to the bash script in order to make it fail faster in case of an error;
3. It creates the convenience composer script gen-ssl, making it even easier than before.

Script flags:
-e: Exit immediately if a pipeline, which may consist of a single simple command, a list, or a compound command returns a non-zero status. reference

-o pipefail: If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. reference

Why?

The idea behind this PR is to streamline the newcomer contributor experience.

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• Documentation pull request created (if not simply a bugfix)

^ No changes were made to the code itself, thus I don't think I should check any of those.

[joelwurtz]

can you rebase ?

[flavioheleno]

done @joelwurtz

[dbu]

thanks a lot. found this while checking open merge requests and cherry-picked it into #64. nice cleanup!