Module always chooses default-critical.css

I think it's actually replacing any found file in CriticalCssProvider.php with whatever comes after that file in the list, even if that is a value of FALSE due to a failed file_get_contents().

Breaking out of the loop when a file is found fixes it:

diff --git a/src/Asset/CriticalCssProvider.php b/src/Asset/CriticalCssProvider.php
index 052b31a..92e40ab 100644
--- a/src/Asset/CriticalCssProvider.php
+++ b/src/Asset/CriticalCssProvider.php
@@ -195,7 +195,10 @@ public function getCriticalCss() {
       }
       else {
         $this->criticalCss = @file_get_contents($this->matchedFilePath);
+        if ($this->criticalCss) {
           $this->criticalCss = trim($this->criticalCss);
+          break;
+        }
       }
     }

It appears to also fail for local files when CSS preprocessing is enabled.

In that case, the call to file_get_contents() only occurs if the file is external.

Disregard my last comment about preprocessing CSS.

It appears that the blank output I'm getting is because the only content of the test CSS files are comments, which are getting stripped by the optimizer.

This MR makes the test CriticalCssHeadTest pass, and I verified manually.

Also tested manually with external asset URLs for #3454343: Add support for downloading critical CSS from an URL.