fix default-backtrace-ice test

when running `tests/ui/panics/default-backtrace-ice.rs locally it gave this error:
```
failures:

---- [ui] tests/ui/panics/default-backtrace-ice.rs stdout ----
Saved the actual stderr to "/home/jyn/src/rust3/build/x86_64-unknown-linux-gnu/test/ui/panics/default-backtrace-ice/default-backtrace-ice.stderr"
diff of stderr:

7
8	aborting due to `-Z treat-err-as-bug=1`
9	stack backtrace:
-	(end_short_backtrace)
-	(begin_short_backtrace)
-	(end_short_backtrace)
-	(begin_short_backtrace)
+	      [... omitted 22 frames ...]
+
```

this is a regression from setting RUST_BACKTRACE=1 by default. we need to turn off the new behavior when running UI tests so that they reflect our dist compiler. normally that's done by checking `sess.unstable_opts.ui_testing`, but this happens extremely early in the compiler before we've expanded arg files. do an extremely simple hack that doesn't work in all cases - we don't need it to work in all cases, only when running UI tests.

compiler/rustc_driver_impl/src/lib.rs

@@ -1388,7 +1388,9 @@ pub fn install_ice_hook(
     // opt in to less-verbose backtraces by manually setting "RUST_BACKTRACE"
     // (e.g. `RUST_BACKTRACE=1`)
     if env::var_os("RUST_BACKTRACE").is_none() {
-        if env!("CFG_RELEASE_CHANNEL") == "dev" {
+        // HACK: this check is extremely dumb, but we don't really need it to be smarter since this should only happen in the test suite anyway.
+        let ui_testing = std::env::args().any(|arg| arg == "-Zui-testing");
+        if env!("CFG_RELEASE_CHANNEL") == "dev" && !ui_testing {
             panic::set_backtrace_style(panic::BacktraceStyle::Short);
         } else {
             panic::set_backtrace_style(panic::BacktraceStyle::Full);

tests/ui/panics/default-backtrace-ice.rs

@@ -1,6 +1,8 @@
 //@ unset-rustc-env:RUST_BACKTRACE
 //@ compile-flags:-Z treat-err-as-bug=1
 //@ error-pattern:stack backtrace:
+// Verify this is a full backtrace, not a short backtrace.
+//@ error-pattern:__rust_begin_short_backtrace
 //@ failure-status:101
 //@ ignore-msvc
 //@ normalize-stderr-test: "note: .*" -> ""

tests/ui/panics/default-backtrace-ice.stderr

@@ -1,5 +1,5 @@
 error: internal compiler error[E0425]: cannot find value `missing_ident` in this scope
-  --> $DIR/default-backtrace-ice.rs:21:13
+  --> $DIR/default-backtrace-ice.rs:23:13
    |
 LL | fn main() { missing_ident; }
    |             ^^^^^^^^^^^^^ not found in this scope