testing: use "exit code" in documentation consistently

The documentation for m.Run says it returns an "exit code" to pass to
os.Exit. The argument to os.Exit is named "code".

While "exit code", "exit status" and "exit status code" are all valid ways
to refer to the same concept, prefer to stick to one form for consistency
and to avoid confusing users.

src/testing/testing.go

@@ -218,7 +218,7 @@
 // then the generated test will call TestMain(m) instead of running the tests
 // directly. TestMain runs in the main goroutine and can do whatever setup
 // and teardown is necessary around a call to m.Run. m.Run will return an exit
-// status that may be passed to os.Exit. If TestMain returns, the test wrapper
+// code that may be passed to os.Exit. If TestMain returns, the test wrapper
 // will pass the result of m.Run to os.Exit itself. When TestMain is called,
 // flag.Parse has not been run. If TestMain depends on command-line flags,
 // including those of the testing package, it should call flag.Parse explicitly.