`x test library/core` stage is off-by-one from its own internal count

[jyn514]

fyi this introduced an off-by-one bug for libcore:

Testing stage2 {core} (aarch64-apple-darwin)
    Finished `release` profile [optimized + debuginfo] target(s) in 0.01s
     Running tests/lib.rs (build/aarch64-apple-darwin/stage1-std/aarch64-apple-darwin/release/deps/coretests-7cd4d959a4810809)

Originally posted by @jyn514 in #146076 (comment)

@rustbot label T-bootstrap C-bug

[Kobzol]

Should be fixed by #149724. Thanks for reporting it!