Rollup merge of #27935 - mbrubeck:cfg-test, r=steveklabnik

r? @steveklabnik

Author: steveklabnik

src/doc/reference.md

@@ -2073,6 +2073,7 @@ The following configurations must be defined by the implementation:
 * `target_pointer_width = "..."`. Target pointer width in bits. This is set
   to `"32"` for targets with 32-bit pointers, and likewise set to `"64"` for
   64-bit pointers.
+* `test`. Enabled when compiling the test harness (using the `--test` flag).
 * `unix`. See `target_family`.
 * `windows`. See `target_family`.