Add docs for compare-output-lines-by-subset flag (#1677)

Author: Veykril

src/tests/headers.md

@@ -52,6 +52,8 @@ found in [`header.rs`] from the compiletest source.
     * [`stderr-per-bitwidth`](ui.md#output-comparison) — separate output per bit width
     * [`dont-check-compiler-stderr`](ui.md#output-comparison) — don't validate stderr
     * [`dont-check-compiler-stdout`](ui.md#output-comparison) — don't validate stdout
+    * [`compare-output-lines-by-subset`](ui.md#output-comparison) — checks output by
+      line subset
 * [Building auxiliary crates](compiletest.md#building-auxiliary-crates)
     * `aux-build`
     * `aux-crate`

src/tests/ui.md

@@ -84,6 +84,9 @@ check for output files:
   [Normalization](#normalization)).
 * `dont-check-compiler-stderr` — Ignores stderr from the compiler.
 * `dont-check-compiler-stdout` — Ignores stdout from the compiler.
+* `compare-output-lines-by-subset` — Checks that the output contains the
+  contents of the stored output files by lines opposed to checking for strict
+  equality.
 
 UI tests run with with `-Zdeduplicate-diagnostics=no` flag which disables
 rustc's built-in diagnostic deduplication mechanism.