Rename integration-testing to interop-testing

"Interoperability" is a more appropriate name for the tests, since they
are used for testing across different implementations. They will do a
bit of integration testing, like for auth, but this is a smaller scale.

It seems the other languages (Go, C++, Node, PHP, Python, Ruby) are
using "interop" to describe the tests, and the test case specifications
document is named with "interop". After this change, C# will be the only
language calling them "integration" tests.

This change just renames the folder and artifact. We can change the
internal package names later. However, once we do a release, old
artifact names will live forever in Maven Central.
diff --git a/settings.gradle b/settings.gradle
index be6bdca..27d128e 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -7,7 +7,7 @@
 include ":grpc-protobuf-nano"
 include ":grpc-netty"
 include ":grpc-testing"
-include ":grpc-integration-testing"
+include ":grpc-interop-testing"
 include ":grpc-all"
 include ":grpc-benchmarks"
 include ":grpc-examples"
@@ -20,7 +20,7 @@
 project(':grpc-protobuf-nano').projectDir = "$rootDir/protobuf-nano" as File
 project(':grpc-netty').projectDir = "$rootDir/netty" as File
 project(':grpc-testing').projectDir = "$rootDir/testing" as File
-project(':grpc-integration-testing').projectDir = "$rootDir/integration-testing" as File
+project(':grpc-interop-testing').projectDir = "$rootDir/interop-testing" as File
 project(':grpc-all').projectDir = "$rootDir/all" as File
 project(':grpc-benchmarks').projectDir = "$rootDir/benchmarks" as File
 project(':grpc-examples').projectDir = "$rootDir/examples" as File