Move generated test code for Google.Protobuf.Test to a separate lib

[ObsidianMinor]

Many of the recent issues with proto2 extensions were accidentally caused by tests passing when code-gen didn't actually compile outside of internal visibility which the Google.Protobuf.Test project has. To prevent mistakes like this in the future, this separates the code-gen from the test project to:

• Keep the InternalAccessVisibleTo attribute out of the test code-gen
• Allow us to use a separate minimum framework and standard version when compiling the code-gen

/cc: @jtattermusch

[jtattermusch]

Test failure:
https://source.cloud.google.com/results/invocations/95078627-231e-41e9-bc4b-665e6e48a041/log
Unittest.cs(891,162): error CS7036: There is no argument given that corresponds to the required formal parameter 'index' of 'Encoding.GetString(byte[], int, int)' [T:\src\github\protobuf\csharp\src\Google.Protobuf.Test.CodeGen\Google.Protobuf.Test.CodeGen.csproj]

[ObsidianMinor]

That's fixed with #6828

[ObsidianMinor]

If you want I can move it to this PR

[jtattermusch]

nit: just net45 instead of net451 should be enough?

[jtattermusch]

Looks like C#3 is nearly not enough. I think we can set the LangVersion to 6 in this PR, leave a TODO and take a followup action to make it compatible with C#3 again in a future PR?

Unittest.cs(5142,61): error CS8024: Feature 'expression-bodied property' is not available in C# 3. Please use language version 6 or greater. [/tmp/protobuf/protobuf/csharp/src/Google.Protobuf.Test.CodeGen/Google.Protobuf.Test.CodeGen.csproj]

[ObsidianMinor]

I could also change it in this PR (or another PR and rebase), since it's a pretty simple change.

private ExtensionSet<T> _Extensions => _extensions;

to

private ExtensionSet<T> _Extensions { get { return _extensions; } }

[jtattermusch]

nit: still says "CodeGen" after renaming to TestProtos

typo: s/seperate/separate/

[jtattermusch]

I could also change it in this PR (or another PR and rebase), since it's a pretty simple change.

private ExtensionSet<T> _Extensions => _extensions;

to

private ExtensionSet<T> _Extensions { get { return _extensions; } }

Can we make that change in a separate PR and only use this PR to verify the end result? - It makes things easier to review.
(let's investigate that this is the only problem with C# 3 incompatibility)

[jtattermusch]

I merged #6856, can you update this PR?

[ObsidianMinor]

Rebased 👍 @jtattermusch

[ObsidianMinor]

Missed a few things when renaming project. Should've gotten everything now.

[jtattermusch]

Distcheck is failing, but otherwise looking good.

++ '[' '!' -z 'csharp/src/Google.Protobuf.Test.TestProtos/Google.Protobuf.Test.TestProtos.csproj ' ']'
++ echo 'Missing files in EXTRA_DIST: csharp/src/Google.Protobuf.Test.TestProtos/Google.Protobuf.Test.TestProtos.csproj '
Missing files in EXTRA_DIST: csharp/src/Google.Protobuf.Test.TestProtos/Google.Protobuf.Test.TestProtos.csproj
++ exit 1

[ObsidianMinor]

Done @jtattermusch

[jtattermusch]

LGTM.