Auto merge of rust-lang#82350 - ehuss:test-chapter, r=jyn514

Add a chapter on the test harness.

There isn't really any online documentation on the test harness, so this adds a chapter to the rustc book which provides information on how the harness works and details on the command-line options.

Author: bors

test/src/cli.rs

@@ -51,14 +51,8 @@ fn optgroups() -> getopts::Options {
         .optflag("", "test", "Run tests and not benchmarks")
         .optflag("", "bench", "Run benchmarks instead of tests")
         .optflag("", "list", "List all tests and benchmarks")
-        .optflag("h", "help", "Display this message (longer with --help)")
-        .optopt(
-            "",
-            "logfile",
-            "Write logs to the specified file instead \
-             of stdout",
-            "PATH",
-        )
+        .optflag("h", "help", "Display this message")
+        .optopt("", "logfile", "Write logs to the specified file", "PATH")
         .optflag(
             "",
             "nocapture",