Include the mode in compiletest's aux-build directory.

The run-pass and pretty run-pass tests could run concurrently, and if they
do, they need to keep their output segregated.

This change might be overkill. We need the suffix for the `pretty` mode,
but we might not need it otherwise. The `debuginfo-lldb` and
`debuginfo-gdb` modes look like they could also need it, but the current
`tests.mk` file happens not to enable both lldb and gdb at the same time,
for incidental reasons.

src/compiletest/runtest.rs

@@ -1452,7 +1452,7 @@ fn make_out_name(config: &Config, testfile: &Path, extension: &str) -> PathBuf {
 fn aux_output_dir_name(config: &Config, testfile: &Path) -> PathBuf {
     let f = output_base_name(config, testfile);
     let mut fname = f.file_name().unwrap().to_os_string();
-    fname.push("libaux");
+    fname.push(&format!(".{}.libaux", config.mode));
     f.with_file_name(&fname)
 }