implement "only-<platforms>" for test headers

This patch implements "only-<platforms>" for tests headers using which one can
specify just the platforms on which the test should run rather than listing all
the platforms to ignore using "ignore-<platforms>".

This is a fix for issues rust-lang#33581 and rust-lang#47459.

Author: Pulkit07

src/tools/compiletest/src/header.rs

@@ -44,6 +44,8 @@ impl EarlyProps {
             props.ignore =
                 props.ignore ||
                 config.parse_cfg_name_directive(ln, "ignore") ||
+                (config.parse_cfg_prefix(ln, "only") &&
+                !config.parse_cfg_name_directive(ln, "only")) ||
                 ignore_gdb(config, ln) ||
                 ignore_lldb(config, ln) ||
                 ignore_llvm(config, ln);
@@ -564,6 +566,17 @@ impl Config {
         }
     }
 
+    fn parse_cfg_prefix(&self, line: &str, prefix: &str) -> bool {
+        // returns whether this line contains this prefix or not. For prefix
+        // "ignore", returns true if line says "ignore-x86_64", "ignore-arch",
+        // "ignore-andorid" etc.
+        if line.starts_with(prefix) && line.as_bytes().get(prefix.len()) == Some(&b'-') {
+            true
+        } else {
+            false
+        }
+    }
+
     fn parse_name_directive(&self, line: &str, directive: &str) -> bool {
         // Ensure the directive is a whole word. Do not match "ignore-x86" when
         // the line says "ignore-x86_64".