Use scala-partest 1.0-RC4

RC3 had a binary incompatibility that led to a crash in parsing options,
due to:
- https://github.com/scala/scala-partest/commit/9ff138ab4b
- https://github.com/scala/scala/commit/2b1563fa74

Also added sonatype as a repo for resolving partest,
so we don't have to wait for artifacts to synch to maven central.
Merge pull request #2862 from adriaanm/partest-1.0-RC4

Use scala-partest 1.0-RC4
SI-7622 Phase assembly is testable

Fixing hash on nodes makes fault detection deterministic,
which aids testing.

Error messages are shortened and .dot files are dumped
automatically on faults to guard against future flakiness.
SI-7622 -Xgenerate-phase-graph is an info option

-Xgenerate-phase-graph is comparable to -Xshow-phases.

The knowledge about what is an info-only option is refactored
to Settings, which also knows which group of options comprise
the optimiser set.
SI-7622 Phases are enabled or not

Refactor the calculation of the "phase chain" a bit.
In particular, initial and terminal phases are not special
except that they must be head and last.

When done, filter for enabled phases. At this commit,
nobody claims to be disabled.

Additional sanity support of phases settings.
SI-7622 -Xshow-phases can show disabled phases

With -Ydebug, -Xshow-phases will show phases that are skipped
or not enabled.

The code is slightly refactored, so the flags table will also benefit.
SI-7622 Plugins can be not enabled

Plugins can interrogate options and declare themselves not
enabled.  The plugin itself can return false from its init
if the options do not compute.  A plugin phase component
can declare itself not enabled, same as an internal phase.

No one exploits this facility at this commit.
SI-7622 Clean Up Phase Assembly

Let optimiser components and continuations plugin opt-out
when required flags are not set.

Wasted time on a whitespace error in check file, so let
--debug dump the processed check file and its diff.
SI-7622 Scaladoc and error message polish

Restores the verbiage "run right after".