Adjust rustc-print-info-issue-138612.rs

- Document test intent to check for `-Whelp` suggestion if
  `--print=lints` was specified.
- Move this test under `tests/ui/print-request/` and rename it to
  `print-lints-help.rs` to better reflect what it is checking.

Author: jieyouxu

tests/ui/print-request/print-lints-help.rs

@@ -0,0 +1,7 @@
+//! Check that we point to `-Whelp` to guide the user to find the list of lints if the user requests
+//! `--print=lints` (which is not a valid print request).
+
+//@ compile-flags: --print lints
+//@ error-pattern: error: unknown print request: `lints`
+//@ error-pattern: help: use `-Whelp` to print a list of lints
+//@ error-pattern: help: for more information, see the rustc book

tests/ui/rustc-print-info-issue-138612.stderr tests/ui/print-request/print-lints-help.stderr

@@ -1,6 +1,6 @@
 error: unknown print request: `lints`
   |
-  = help: valid print requests are: `all-target-specs-json`, `calling-conventions`, `cfg`, `check-cfg`, `code-models`, `crate-name`, `deployment-target`, `file-names`, `host-tuple`, `link-args`, `native-static-libs`, `relocation-models`, `split-debuginfo`, `stack-protector-strategies`, `sysroot`, `target-cpus`, `target-features`, `target-libdir`, `target-list`, `target-spec-json`, `tls-models`
+  = help: valid print requests are: `all-target-specs-json`, `calling-conventions`, `cfg`, `check-cfg`, `code-models`, `crate-name`, `deployment-target`, `file-names`, `host-tuple`, `link-args`, `native-static-libs`, `relocation-models`, `split-debuginfo`, `stack-protector-strategies`, `supported-crate-types`, `sysroot`, `target-cpus`, `target-features`, `target-libdir`, `target-list`, `target-spec-json`, `tls-models`
   = help: use `-Whelp` to print a list of lints
   = help: for more information, see the rustc book: https://doc.rust-lang.org/rustc/command-line-arguments.html#--print-print-compiler-information