Adds support for Musl

[t089]

Resolves #143.

This adds support for building the project with the "Swift Linux SDK".

For this imports of Glibc needs to be conditionalized for cases where musl is available.

Also there are some small API differences between musl and Glibc.

Also the target "OTelTesting" should be a testTarget otherwise it will be built together with the rest when invoking swift build. This fails though because the Swift Linux SDK does not contain any testing framework.

Also removes a seemingly unused import Foundation.

[t089]

Just realized that this is a duplicate of #148. The approach taken here is a bit different though:

• This PR avoids the import of Foundation for OTelEnvironment
• Instead of conditionally (not) compiling OTelTesting, this PR converts it into a testTarget so that it is only part of the "testing" build.

@lovetodream what do you think?

[lovetodream]

• This PR avoids the import of Foundation for OTelEnvironment

In my PR, I've replaced environ with ProcessInfo.processInfo.environment (due to thread safety and no need for different implementations in glibc and musl), Hummingbird did the same some time ago. In that version the Foundation import would still be needed.

• Instead of conditionally (not) compiling OTelTesting, this PR converts it into a testTarget so that it is only part of the "testing" build.

I am not sure if this is the right thing to do, as OTelTesting is not really a test target. It is more like a support library for writing tests for OTel (similar to NIOTestUtils in swift-nio), at least that's my understanding of it.

At the end, both approaches achieve the same goal, so either one is fine for me 😄
Let's see what @slashmo thinks about it.

[t089]

I am not sure if this is the right thing to do, as OTelTesting is not really a test target. It is more like a support library for writing tests for OTel (similar to NIOTestUtils in swift-nio), at least that's my understanding of it.

Yeah, fair. But NIOTestUtils does not depend on any testing framework. If you want to depend on a testing framework it needs to be a testTarget.