dmd testsuite's runner is now in D

[Seb]

Just a quick heads-up for all the people using Windows and who 
have previously complained about the usage of GNUmake for DMD's 
testsuite.

Since a few days, all you need to run the testsuite is a DMD 
compiler.
The test/run.d has a few handy features, so check out the new 
documentation:

https://github.com/dlang/dmd/blob/master/test/README.md

The quick summary:

./run                # runs all test (with all available threads)
./run -j1            # runs all test (with a single thread)
./run.d fail         # runs all fail_compilation tests
./run.d compilable   # runs all compilable tests
./run.d fail_compilation/diag10089.d  # runs a single test
./run.d fail_compilation/diag*.d      # if you shell supports 
expansion, you can build your own subset of tests to run
./run.d fail AUTO_UPDATE=1            # automatically update the 
TEST_OUTPUT text (and generated files soon)

It's intended as a drop-in replacement for the Makefile, so all 
previous commands should work and it can be invoked from anywhere 
(no need for -C).

There are still a few bash tests that might fail on Windows, so 
use e.g. your git shell [2].

Disclaimer: test/run.d is currently still experimental. If you 
run into any issues, please report them.

PS: The plan is to slowly transition from the Makefile to 
test/run.d, so any issues you have with test/run.d are important.

[1] https://github.com/dlang/dmd/pull/8215
[2] https://gitforwindows.org