test: index shards at 1, not 0

This has no tests because it's near impossible to test -- since TestFn uses
`proc`s, they can not be cloned or tested for equality. The only way to really
test this is making sure that for a given number of shards `a`, sharding from
1 to `a` yields the complete set of tests. But `filter_tests` takes its vector
by value and `proc`s cannot be compared.

[breaking-change]

Closes #10898

src/test/run-make/test-shard-completeness/Makefile

@@ -0,0 +1,7 @@
+-include ../tools.mk
+
+all:
+	# Running all the shards should hit every test
+	$(RUSTC) --test main.rs
+	$(call RUN,main) --test-shard 1.2 | grep "test_1 ... ok"
+	$(call RUN,main) --test-shard 2.2 | grep "test_2 ... ok"

src/test/run-make/test-shard-completeness/main.rs

@@ -0,0 +1,16 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![crate_type = "lib"]
+
+#[test]
+fn test_1() { }
+#[test]
+fn test_2() { }