Fix Homebrew style: Use assert_path_exists instead of assert_predicate :exist?

Homebrew's style checker requires using assert_path_exists for file existence
checks instead of assert_predicate with :exist?. Keep assert_predicate only
for the :executable? checks.

Author: sbryngelson

packaging/homebrew/mfc.rb

@@ -59,15 +59,15 @@ def caveats
 
   test do
     # Test that the binaries exist and are executable
-    assert_predicate bin/"pre_process", :exist?
+    assert_path_exists bin/"pre_process"
     assert_predicate bin/"pre_process", :executable?
-    assert_predicate bin/"simulation", :exist?
+    assert_path_exists bin/"simulation"
     assert_predicate bin/"simulation", :executable?
-    assert_predicate bin/"post_process", :exist?
+    assert_path_exists bin/"post_process"
     assert_predicate bin/"post_process", :executable?
 
     # Verify examples were installed
-    assert_predicate pkgshare/"examples", :exist?
-    assert_predicate pkgshare/"examples/1D_sodshocktube/case.py", :exist?
+    assert_path_exists pkgshare/"examples"
+    assert_path_exists pkgshare/"examples/1D_sodshocktube/case.py"
   end
 end