[clang-doc] Avoid reading files in unit tests (#141269)

In #138062 it was brought up that this was an anti-pattern. We'll need
to Migrate all of the mustache unittests that need to read template 
files to lit tests, and disable them until tool support lands.

Author: ilovepi

clang-tools-extra/unittests/clang-doc/HTMLMustacheGeneratorTest.cpp

@@ -91,10 +91,12 @@ TEST(HTMLMustacheGeneratorTest, generateDocs) {
   unittest::TempDir RootTestDirectory("generateDocsTest", /*Unique=*/true);
   CDCtx.OutDirectory = RootTestDirectory.path();
 
-  getMustacheHtmlFiles(CLANG_DOC_TEST_ASSET_DIR, CDCtx);
+  // FIXME: We can't read files during unit tests. Migrate to lit once
+  // tool support lands.
+  // getMustacheHtmlFiles(CLANG_DOC_TEST_ASSET_DIR, CDCtx);
 
   EXPECT_THAT_ERROR(G->generateDocs(RootTestDirectory.path(), {}, CDCtx),
-                    Succeeded())
+                    Failed())
       << "Failed to generate docs.";
 }