Add options for parsing test files (#942)

* Add options for parsing test files

This change allows adding parsing parameters more easily; e.g. a parameter is needed for setting the default calling convention for functions parsed as a part of the `run` test feature.

* Set default calling convention that of the host for `test run` file tests

Previously `test run` used the parser's hard-coded CallConv::Fast as the default calling convention but with this change any test being `run` will use the default calling convention of the machine running the test. `test run` will now throw an error if the calling convention of the function does not match the host's.

cranelift/reader/src/sourcemap.rs

@@ -211,7 +211,7 @@ impl SourceMap {
 
 #[cfg(test)]
 mod tests {
-    use crate::parse_test;
+    use crate::{parse_test, ParseOptions};
 
     #[test]
     fn details() {
@@ -222,8 +222,7 @@ mod tests {
                              ebb0(v4: i32, v7: i32):
                                v10 = iadd v4, v7
                              }",
-            None,
-            None,
+            ParseOptions::default(),
         )
         .unwrap();
         let map = &tf.functions[0].1.map;