Mute testMessageForPemKeyOutsideConfigDir on Windows (#76234)

... from the SSLErrorMessageFileTests suite.

Relates #76166

Author: dimitris-athanasiou

x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/ssl/SSLErrorMessageFileTests.java

@@ -119,6 +119,7 @@ public void testMessageForPemCertificateOutsideConfigDir() throws Exception {
     }
 
     public void testMessageForPemKeyOutsideConfigDir() throws Exception {
+        assumeFalse("https://github.com/elastic/elasticsearch/issues/76166", Constants.WINDOWS);
         checkBlockedKeyManagerResource("PEM private key", "key", withCertificate("cert1a.crt"));
     }