On non-compiletest tests, only use a single test task when valgrinding

[brson]

For test runners that don't spawn subprocesses for every test (rustc, core, std, rustdoc), trying to run tests in parallel under valgrind just makes the entire test run slower. The build should detect when we are valgrinding and pass a flag (that doesn't exist yet) to the test runner telling it to use only 1 test task at a time.

I've noticed that the linux1 bot is taking an absurd amount of time to run coretest. This might help.

[emberian]

Triage bump; still relevant.

[brson]

There's a function exported from the runtime to check whether we're running under valgrind that could be used for this.

[thestinger]

@brson: did we ever do this?

[brson]

@thestinger no

[catamorphism]

Low, no milestone

[alexcrichton]

Closing, this is done for all pieces of infrastructure using libtest: https://github.com/mozilla/rust/blob/master/src/libstd/rt/util.rs#L49-L72