Allow multiple threads and panicking tests when testing regex

I guess this was a leftover from very early in the development of
cg_clif. Allowing multiple threads significantly improves performance,
while panicking tests can run now thanks to -Zpanic-abort-tests.

build_system/tests.rs

@@ -187,15 +187,7 @@ const fn jit_bin(config: &'static str, source: &'static str, args: &'static str)
 
         if runner.is_native {
             let mut run_cmd = REGEX.test(&runner.target_compiler, &runner.dirs);
-            run_cmd.args([
-                "--tests",
-                "--",
-                "--exclude-should-panic",
-                "--test-threads",
-                "1",
-                "-Zunstable-options",
-                "-q",
-            ]);
+            run_cmd.args(["--tests", "--", "-q"]);
             spawn_and_wait(run_cmd);
         } else {
             eprintln!("Cross-Compiling: Not running tests");