Generate C# conformance test in script (#11880)

This is already generated code, but wasn't included in our generation script. This change makes sure it will be up-to-date, and uses the now-conventional .pb.cs file extension.

This does *not* affect the conformance tests run from Bazel, as they generate the file separately and build the conformance tests using only Program.cs and the freshly-generated code. However, the csharp/src/Google.Protobuf.Conformance/BUILD.bazel still needs to be kept up-to-date with the filename change.

Closes #11880

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/11880 from jskeet:generate-conformance 34bb22cc46
PiperOrigin-RevId: 509542860

csharp/generate_protos.sh

@@ -79,3 +79,8 @@ $PROTOC -Isrc -I. \
 $PROTOC -Iexamples -Isrc --csharp_out=csharp/src/AddressBook \
     --csharp_opt=file_extension=.pb.cs \
     examples/addressbook.proto
+
+# Conformance tests
+$PROTOC -I. --csharp_out=csharp/src/Google.Protobuf.Conformance \
+    --csharp_opt=file_extension=.pb.cs \
+    conformance/conformance.proto

csharp/src/Google.Protobuf.Conformance/BUILD.bazel

@@ -37,7 +37,7 @@ CONFORMANCE_DEPS = [
 filegroup(
     name = "srcs",
     srcs = [
-        "Conformance.cs",
+        "Conformance.pb.cs",
         "Program.cs",
         "Google.Protobuf.Conformance.csproj",
     ],
@@ -88,7 +88,7 @@ pkg_files(
     srcs = [
         "BUILD.bazel",
         "Google.Protobuf.Conformance.csproj",
-        "Conformance.cs",
+        "Conformance.pb.cs",
         "Program.cs",
     ],
     strip_prefix = strip_prefix.from_root(""),