audit: tweak assert/refute_predicate audit

Addresses some of the issues I raised in https://github.com/Homebrew/homebrew-core/pull/19127#issue-263566817.
Would still like to work out a way to check `File.exist?`-style language
inside the `test do` block exclusively but for now it's better to be
too conservative on the audit than to be overly-zealous and flag too
much.

Library/Homebrew/dev-cmd/audit.rb

@@ -967,12 +967,12 @@ class FormulaAuditor
       problem "Use `assert_match` instead of `assert ...include?`"
     end
 
-    if line =~ /(assert File\.exist\?|File\.exist\?)/
+    if line =~ /(assert File\.exist\?|assert \(.*\)\.exist\?)/
       problem "Use `assert_predicate <path_to_file>, :exist?` instead of `#{Regexp.last_match(1)}`"
     end
 
-    if line =~ /(assert !File\.exist\?|!File\.exist\?)/
-      problem "Use `refute_predicate <path_to_file>, :exist?` instead of `#{Regexp.last_match(1)}`"
+    if line =~ /assert !File\.exist\?/
+      problem "Use `refute_predicate <path_to_file>, :exist?` instead of `assert !File.exist?`"
     end
 
     return unless @strict