Factor test toolchain

[smparkes]

Please look at #962 and consider earlier uncommitted PRs first. This PR is based on those PRs and will give you smaller PRs to review.

Description

• Create a new test toolchain with a provider in the normal pattern
• Move scalatest jars/dependencies to the toolchain
• Move scalatest flags to the toolchain
• Tiny bit of cleanup (don't "_" prefix in BUILD files since they can't be loaded)

Motivation

Multiple scala version builds within the same bazel run #962

[andyscott]

This looks super neat. A few comments:

• can we rename bootstrap_toolchain to scala_toolchain?
• can we rename test_toolchain to scala_test_toolchain?
• can we update the jar field to instead be deps and accept multiple JavaInfo targets? This wills likely scale better. In particular I can imagine pushing this a bit further to work well with Generic scala_test rule #959 for easily configuring test framework deps.

[smparkes]

@andyscott Think you may want to look at #978 first. More reviewable since it's a smaller chunk. I'll respond to some of the comments there since they're in the earlier PRs.

can we rename test_toolchain to scala_test_toolchain?

I've been wondering about this. It's been unclear to me how much this is any scala test or scalatest. We obviously provide the scalatest (and scalactic? ... haven't gotten there yet ...) jars automatically.

In #959, is the idea still fundamentally scalatest with extensions or is it "a scala test" for some framework ...

can we update the jar field to instead be deps and accept multiple JavaInfo targets?

Yup.

[andyscott]

#959 is a scala_test rule that behaves like SBT's test command: it runs whatever tests can be discovered by any testing frameworks on the classpath. It's not specific to Scalatest, Specs2, uTest, Minitest, etc.

Coupled with some cleanup to toolchains it would be possible to configure the generic #959 scala_test rule to behave exactly like the current scala_test rule. The toolchain would configure the Scalatest framework jars and then the test rule would only run Scalatest tests. A user, of course, could configure Scalacheck and Scalatest jars, and then the test rules would discover tests from both Scalacheck and Scalatest.

[smparkes]

Updated the toolchain name and the jar -> deps thing.

The other naming is pending ...

[smparkes]

At this point, there's not enough need/support for this (including myself) to continue forward.