[clang-tools-extra] [clang-doc] Add basic e2e test (PR #93928)

[Paul Kirth via cfe-commits]

================
@@ -0,0 +1,17 @@
+[
+  {
+    "directory": "$test_dir/build",
----------------
ilovepi wrote:

Would the following work? It would be really nice if we could make this operate w/o the `sed` command.
```suggestion
    "directory": "./build",
```
or 
```suggestion
    "directory": ".",
```

https://github.com/llvm/llvm-project/pull/93928