TestFile preamble comments apply to all functions.

Include the test file preamble comments when building a filecheck
instance for every function in the file.

This makes it possible to define common regex variables in the preamble
and use these definitions for all the functions.

cranelift/src/filetest/runone.rs

@@ -50,6 +50,7 @@ pub fn run(path: &Path) -> TestResult {
 
     for (func, details) in testfile.functions {
         let mut context = Context {
+            preamble_comments: &testfile.preamble_comments,
             details: details,
             verified: false,
             flags: flags,