add test for describe cacheArtifactPath

test/pr2644-describe-artifact-path/.gitignore

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+dubhome/
+pr2644-describe-artifact-path

test/pr2644-describe-artifact-path/dub.sdl

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+name "pr2644-describe-artifact-path";
+targetType "executable";

test/pr2644-describe-artifact-path/source/describe_artifact_path.d

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+module describe_artifact_path;
+
+import std.path;
+import std.file;
+import std.process;
+import std.stdio;
+import std.json;
+
+void main()
+{
+    const dubhome = __FILE_FULL_PATH__.dirName().dirName().buildNormalizedPath("dubhome");
+    if (exists(dubhome))
+    {
+        rmdirRecurse(dubhome);
+    }
+    scope (success)
+    {
+        // leave dubhome in the tree for analysis in case of failure
+        //rmdirRecurse(dubhome);
+    }
+
+    const string[string] env = [
+        "DUB_HOME": dubhome,
+    ];
+    const fetchProgram = [
+        environment["DUB"],
+        "fetch",
+        "[email protected]",
+    ];
+    auto dubFetch = spawnProcess(fetchProgram, stdin, stdout, stderr, env);
+    wait(dubFetch);
+
+
+    const describeProgram = [
+        environment["DUB"],
+        "describe",
+        "--compiler=" ~ environment["DC"],
+        "--build=debug",
+        "--config=lib",
+        "[email protected]",
+    ];
+    auto result = execute(describeProgram, env);
+    assert(result.status == 0, "expected dub describe to return zero");
+    auto json = parseJSON(result.output);
+
+    auto cacheFile = json["targets"][0]["cacheArtifactPath"].str;
+    assert(!exists(cacheFile), "found cache file in virgin dubhome");
+
+    const buildProgram = [
+        environment["DUB"],
+        "build",
+        "--compiler=" ~ environment["DC"],
+        "--build=debug",
+        "--config=lib",
+        "[email protected]",
+    ];
+    auto dubBuild = spawnProcess(buildProgram, stdin, stdout, stderr, env);
+    wait(dubBuild);
+
+    assert(exists(cacheFile), "did not find cache file after build");
+}