Auto merge of #45748 - petrochenkov:short, r=alexcrichton

Shorten paths to auxiliary files created by tests

I'm hitting issues with long file paths to object files created by the test suite, similar to rust-lang/rust#45103 (comment).

If we look at the object file path in rust-lang/rust#45103 we can see that the patch contains of few components:
```
specialization-cross-crate-defaults.stage2-x86_64-pc-windows-gnu.run-pass.libaux\specialization_cross_crate_defaults.specialization_cross_crate_defaults0.rust-cgu.o
```
=>

1. specialization-cross-crate-defaults // test name, required
2. stage2 // stage disambiguator, required
3. x86_64-pc-windows-gnu // target disambiguator, required
4. run-pass // mode disambiguator, rarely required
5. libaux // suffix, can be shortened
6. specialization_cross_crate_defaults // required, there may be several libraries in the directory
7. specialization_cross_crate_defaults0 // codegen unit name, can be shortened?
8. rust-cgu // suffix, can be shortened?
9. o // object file extension

This patch addresses items `4`, `5` and `8`.
`libaux` is shortened to `aux`, `rust-cgu` is shortened to `rcgu`, mode disambiguator is omitted unless it's necessary (for pretty-printing and debuginfo tests, see rust-lang/rust@38d26d8)

I haven't touched names of codegen units though (`specialization_cross_crate_defaults0`).
Is it useful for them to have descriptive names including the crate name, as opposed to just `0` or `cgu0` or something?

src/common.rs

@@ -34,6 +34,20 @@ pub enum Mode {
     MirOpt,
 }
 
+impl Mode {
+    pub fn disambiguator(self) -> &'static str {
+        // Run-pass and pretty run-pass tests could run concurrently, and if they do,
+        // they need to keep their output segregated. Same is true for debuginfo tests that
+        // can be run both on gdb and lldb.
+        match self {
+            Pretty => ".pretty",
+            DebugInfoGdb => ".gdb",
+            DebugInfoLldb => ".lldb",
+            _ => "",
+        }
+    }
+}
+
 impl FromStr for Mode {
     type Err = ();
     fn from_str(s: &str) -> Result<Mode, ()> {

src/runtest.rs

@@ -1565,7 +1565,7 @@ actual:\n\
     fn aux_output_dir_name(&self) -> PathBuf {
         let f = self.output_base_name();
         let mut fname = f.file_name().unwrap().to_os_string();
-        fname.push(&format!(".{}.libaux", self.config.mode));
+        fname.push(&format!("{}.aux", self.config.mode.disambiguator()));
         f.with_file_name(&fname)
     }