Build IntegrationTest package as part of CI pipeline (#56)

### Motivation

We have an package in a subdirectory in this repo that we can be used as
an
integration test to gate changes to the various repos in Swift OpenAPI
project,
including the runtime package.

This will likely be implemented by shallow cloning this repo in the pull
request pipelines of other projects and running a script, using `swift
package
edit` to override the dependency on the package being tested, and
building the
integration test package.

### Modifications

- Simplify the integration test package location and name.
- Add a CI script that runs the integration test with a package
override.
- Run this script as part of the soundness pipeline.

### Result

On each pull request, the integration test package will be built with
the
changes proposed in the pull request.

### Test Plan

The CI pipeline for this PR will run the integration test because it's
been
added to the soundness script, which is run as part of an existing CI
pipeline.

I have also validated this locally:

```console
❯ docker-compose -f docker/docker-compose.yaml run --build soundness
...
** Running /code/scripts/check-for-broken-symlinks.sh...
** Checking for broken symlinks...
** ✅ Found 0 symlinks.
** Running /code/scripts/check-for-unacceptable-language.sh...
** Checking for unacceptable language...
** ✅ Found no unacceptable language.
** Running /code/scripts/check-license-headers.sh...
** ✅ Found no files with missing license header.
** Running /code/scripts/run-swift-format.sh...
** ✅ Ran swift-format with no errors.
** Running /code/scripts/check-for-docc-warnings.sh...
...
** ✅ Generated documentation with no warnings.
** Running /code/scripts/run-integration-test.sh...
** Checking required executables...
...
Build complete! (42.53s)
** ✅ Successfully built integration test package.
** ✅ All soundness check(s) passed.
```

### Notes

This PR adds the integration test to the soundness script, but we
probably want
to split this out into its own pipeline.

---------

Signed-off-by: Si Beaumont <[email protected]>

Author: simonjbeaumont

IntegrationTests/Simple/.gitignore renamed to IntegrationTest/.gitignore

@@ -15,7 +15,7 @@
 import PackageDescription
 
 let package = Package(
-    name: "Simple",
+    name: "swift-openapi-integration-test",
     platforms: [
         .macOS(.v13)
     ],