C#: Rename integration test directories.

We are no longer bound to the platform-specific directories, so simplify the test organization.
If you don't want this change, just skip merging this PR. It's purely optional.

This is not very invasive for C#, I'm just dropping the `only`
suffix. You could also merge all the platform-specific test dirs,
or all test dirs into the top-level directory. I'll leave that up to you.

csharp/ql/integration-tests/windows/diag_multiple_scripts/build.bat

@@ -0,0 +1 @@
+exit /b 1

csharp/ql/integration-tests/windows/diag_multiple_scripts/diagnostics.expected

@@ -0,0 +1,28 @@
+{
+  "markdownMessage": "CodeQL could not find any project or solution files in your repository.\n\nSet up a [manual build command](https://docs.github.com/en/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-the-codeql-workflow-for-compiled-languages).",
+  "severity": "error",
+  "source": {
+    "extractorName": "csharp",
+    "id": "csharp/autobuilder/no-projects-or-solutions",
+    "name": "No project or solutions files found"
+  },
+  "visibility": {
+    "cliSummaryTable": true,
+    "statusPage": true,
+    "telemetry": true
+  }
+}
+{
+  "markdownMessage": "CodeQL found multiple potential build scripts for your project and attempted to run `build.bat`, which failed. This may not be the right build script for your project.\n\nSet up a [manual build command](https://docs.github.com/en/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-the-codeql-workflow-for-compiled-languages).",
+  "severity": "error",
+  "source": {
+    "extractorName": "csharp",
+    "id": "csharp/autobuilder/multiple-build-scripts",
+    "name": "There are multiple potential build scripts"
+  },
+  "visibility": {
+    "cliSummaryTable": true,
+    "statusPage": true,
+    "telemetry": true
+  }
+}

csharp/ql/integration-tests/windows/diag_multiple_scripts/scripts/build.bat

@@ -0,0 +1 @@
+exit /b 1

csharp/ql/integration-tests/windows/diag_multiple_scripts/test.py

@@ -0,0 +1,6 @@
+import runs_on
+
+
+@runs_on.windows
+def test(codeql, csharp):
+    codeql.database.create(_assert_failure=True)