Replace test usages of DigestHashFunction#getDefaultUnchecked and implicit DigestHashFunction#getDefault calls by the actual default hash function. Also remove a bunch of unnecessary Clock injections.

PiperOrigin-RevId: 330572846
diff --git a/src/test/java/com/google/devtools/build/lib/vfs/PathFragmentTest.java b/src/test/java/com/google/devtools/build/lib/vfs/PathFragmentTest.java
index 3440e83..763d2b4 100644
--- a/src/test/java/com/google/devtools/build/lib/vfs/PathFragmentTest.java
+++ b/src/test/java/com/google/devtools/build/lib/vfs/PathFragmentTest.java
@@ -43,7 +43,7 @@
 
   @Test
   public void testEqualsAndHashCode() {
-    InMemoryFileSystem filesystem = new InMemoryFileSystem();
+    InMemoryFileSystem filesystem = new InMemoryFileSystem(DigestHashFunction.SHA256);
 
     new EqualsTester()
         .addEqualityGroup(