Auto merge of #7350 - alexcrichton:mock-std, r=ehuss

Improve test suite for `-Zbuild-std`

This commit is aimed directly at rust-lang/wg-cargo-std-aware#33 and in
general making the `-Zbuild-std` tests more robust. The main change here
is that a new source tree is checked in, `tests/testsuite/mock-std`,
which mirrors rust-lang/rust's own tree for libstd. This mock tree is as
empty as it can be, ideally duplicating almost nothing but for not
requiring duplication of Cargo metadata about patches and such.

The end result here looks like:

* All `-Zbuild-std` tests are now run in parallel
* All tests run much more quickly since they're compiling tiny crates
  instead of actually compiling libstd/libcore
* No tests require network access
* We verify that crates have access to the "custom" libraries
  that we build

Coverage of tests is not currently expanded, but it's hoped that we
could add that shortly afterwards. Coverage has actually gone down
slightly since the custom target test was commented out temporarily and
the full integration test of running `-Zbuild-std` isn't run on CI any
more.

Closes rust-lang/wg-cargo-std-aware#33