Unit-test: Add Old expansion test on nix (EASYRSA_FORCE_SAFE_SSL)

[TinCanTech]

This forces the unit test to use the original sed expansion of openssl-easyrsa.cnf on Linux. This also forces use of escape_hazard().

[TinCanTech]

The failure on Linux is identical to the failure on Windows: escape_hazard() passes a file containing export to source_vars(), which fails because use of export is prohibited in a file to be sourced.

Because the file being sourced is generated internally, the simplest solution is to source the file directly, no validation of the data is required.

This will be addressed by https://github.com/OpenVPN/easy-rsa/pull/1150/commits/3277f5182ff31f60f7305eeed48e419481077ed6