Add a centralized test for print request stability gating

I can't find any dedicated tests that actually exercises the stability
gating (via `-Z unstable-options`) of print requests, so here's a
dedicated one.

I coalesced `tests/ui/feature-gates/feature-gate-print-check-cfg.rs`
into this test, because AFAICT that print request is not feature gated,
but only `-Z unstable-options`-gated just like other unstable print
requests.

Author: jieyouxu

tests/ui/feature-gates/feature-gate-print-check-cfg.rs

@@ -0,0 +1,84 @@
+//! Check that we properly gate unstable print requests (`--print=KIND`) and require the user to
+//! specify `-Z unstable-options` to use unstable print requests.
+
+// We don't care about the exact *stdout* output (i.e. what the print requests actually give back)
+// for the purposes of this test.
+//@ dont-check-compiler-stdout
+// We want to check for the core error message of the unstable print requests being `-Z
+// unstable-options`-gated and not the help because the help can change with addition of a new print
+// request, which is not important for the purposes of this test.
+//@ dont-check-compiler-stderr
+
+// =======================
+// Unstable print requests
+// =======================
+
+//@ revisions: all_target_specs_json
+//@[all_target_specs_json] compile-flags: --print=all-target-specs-json
+//@[all_target_specs_json] error-pattern: the `-Z unstable-options` flag must also be passed
+//@ revisions: check_cfg
+//@[check_cfg] compile-flags: --print=check-cfg
+//@[check_cfg] error-pattern: the `-Z unstable-options` flag must also be passed
+//@ revisions: target_spec_json
+//@[target_spec_json] compile-flags: --print=target-spec-json
+//@[target_spec_json] error-pattern: the `-Z unstable-options` flag must also be passed
+
+// =======================
+// Stable print requests
+// =======================
+
+//@ revisions: calling_conventions
+//@[calling_conventions] compile-flags: --print=calling-conventions
+//@[calling_conventions] check-pass
+//@ revisions: cfg
+//@[cfg] compile-flags: --print=cfg
+//@[cfg] check-pass
+//@ revisions: code_models
+//@[code_models] compile-flags: --print=code-models
+//@[code_models] check-pass
+//@ revisions: crate_name
+//@[crate_name] compile-flags: --print=crate-name
+//@[crate_name] check-pass
+//@ revisions: deployment_target
+// Note: `--print=deployment_target` is only accepted on Apple targets.
+//@[deployment_target] only-apple
+//@[deployment_target] compile-flags: --print=deployment-target
+//@[deployment_target] check-pass
+//@ revisions: file_names
+//@[file_names] compile-flags: --print=file-names
+//@[file_names] check-pass
+//@ revisions: host_tuple
+//@[host_tuple] compile-flags: --print=host-tuple
+//@[host_tuple] check-pass
+//@ revisions: link_args
+//@[link_args] compile-flags: --print=link-args
+//@[link_args] check-pass
+//@ revisions: native_static_libs
+//@[native_static_libs] compile-flags: --print=native-static-libs
+//@[native_static_libs] check-pass
+//@ revisions: relocation_models
+//@[relocation_models] compile-flags: --print=relocation-models
+//@[relocation_models] check-pass
+//@ revisions: split_debuginfo
+//@[split_debuginfo] compile-flags: --print=split-debuginfo
+//@[split_debuginfo] check-pass
+//@ revisions: stack_protector_strategies
+//@[stack_protector_strategies] compile-flags: --print=stack-protector-strategies
+//@[stack_protector_strategies] check-pass
+//@ revisions: target_cpus
+//@[target_cpus] compile-flags: --print=target-cpus
+//@[target_cpus] check-pass
+//@ revisions: target_features
+//@[target_features] compile-flags: --print=target-features
+//@[target_features] check-pass
+//@ revisions: target_libdir
+//@[target_libdir] compile-flags: --print=target-libdir
+//@[target_libdir] check-pass
+//@ revisions: target_list
+//@[target_list] compile-flags: --print=target-list
+//@[target_list] check-pass
+//@ revisions: tls_models
+//@[tls_models] compile-flags: --print=tls-models
+//@[tls_models] check-pass
+
+fn main() {}