UI tests for -Z instrument-xray

I'm tired of testing it manually, just codify my expectations in tests.
They're pretty low-maintenance.

src/tools/tidy/src/ui_tests.rs

@@ -9,7 +9,7 @@ use std::path::Path;
 
 const ENTRY_LIMIT: usize = 1000;
 // FIXME: The following limits should be reduced eventually.
-const ROOT_ENTRY_LIMIT: usize = 939;
+const ROOT_ENTRY_LIMIT: usize = 940;
 const ISSUES_ENTRY_LIMIT: usize = 2001;
 
 fn check_entries(path: &Path, bad: &mut bool) {

tests/ui/instrument-xray/flags-always-never-1.rs

@@ -0,0 +1,6 @@
+// Checks that `-Z instrument-xray` does not allow `always` and `never` simultaneously.
+//
+// compile-flags: -Z instrument-xray=always,never
+// error-pattern: incorrect value `always,never` for unstable option `instrument-xray`
+
+fn main() {}

tests/ui/instrument-xray/flags-always-never-1.stderr

@@ -0,0 +1,2 @@
+error: incorrect value `always,never` for unstable option `instrument-xray` - either a boolean (`yes`, `no`, `on`, `off`, etc), or a comma separated list of settings: `always` or `never` (mutually exclusive), `ignore-loops`, `instruction-threshold=N`, `skip-entry`, `skip-exit` was expected
+

tests/ui/instrument-xray/flags-always-never-2.rs

@@ -0,0 +1,8 @@
+// Checks that `-Z instrument-xray` allows `always` and `never` sequentially.
+// (The last specified setting wins, like `-Z instrument-xray=no` as well.)
+//
+// compile-flags: -Z instrument-xray=always
+// compile-flags: -Z instrument-xray=never
+// check-pass
+
+fn main() {}

tests/ui/instrument-xray/flags-basic.rs

@@ -0,0 +1,8 @@
+// Verifies basic `-Z instrument-xray` flags.
+//
+// compile-flags: -Z instrument-xray
+// compile-flags: -Z instrument-xray=skip-exit
+// compile-flags: -Z instrument-xray=ignore-loops,instruction-threshold=300
+// check-pass
+
+fn main() {}

tests/ui/instrument-xray/flags-dupe-always.rs

@@ -0,0 +1,6 @@
+// Checks that `-Z instrument-xray` does not allow duplicates.
+//
+// compile-flags: -Z instrument-xray=always,always
+// error-pattern: incorrect value `always,always` for unstable option `instrument-xray`
+
+fn main() {}

tests/ui/instrument-xray/flags-dupe-always.stderr

@@ -0,0 +1,2 @@
+error: incorrect value `always,always` for unstable option `instrument-xray` - either a boolean (`yes`, `no`, `on`, `off`, etc), or a comma separated list of settings: `always` or `never` (mutually exclusive), `ignore-loops`, `instruction-threshold=N`, `skip-entry`, `skip-exit` was expected
+

tests/ui/instrument-xray/flags-dupe-ignore-loops.rs

@@ -0,0 +1,6 @@
+// Checks that `-Z instrument-xray` does not allow duplicates.
+//
+// compile-flags: -Z instrument-xray=ignore-loops,ignore-loops
+// error-pattern: incorrect value `ignore-loops,ignore-loops` for unstable option `instrument-xray`
+
+fn main() {}

tests/ui/instrument-xray/flags-dupe-ignore-loops.stderr

@@ -0,0 +1,2 @@
+error: incorrect value `ignore-loops,ignore-loops` for unstable option `instrument-xray` - either a boolean (`yes`, `no`, `on`, `off`, etc), or a comma separated list of settings: `always` or `never` (mutually exclusive), `ignore-loops`, `instruction-threshold=N`, `skip-entry`, `skip-exit` was expected
+