testsuite: Fix Windows platform test

Previously we used platform.system() and while this worked fine (e.g.
returned `Windows`, as expected) locally under both msys and MingW64
Python distributions, it inexplicably returned `MINGW64_NT-10.0`
under MingW64 Python on CI. It seems os.name is more reliable so we now
use that instead..
22 jobs for master in 681 minutes and 36 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Lint
passed #232502
lint
lint-linters

00:00:15

passed #232505
lint
lint-submods-branch

00:00:19

passed #232503
lint
lint-testsuite

00:00:18

passed #232504
lint
typecheck-testsuite

00:00:16

 
  Quick Build
passed #232506
x86_64-linux
hadrian-ghc-in-ghci

00:18:21

 
  Build
passed #232507
x86_64-linux
validate-x86_64-linux-deb9-hadrian

03:07:51

 
  Full Build
failed #232510
aarch64-linux allowed to fail
validate-aarch64-linux-deb9

03:24:50

passed #232511
armv7-linux
validate-armv7-linux-deb9

03:49:23

passed #232512
x86_64-linux
validate-i386-linux-deb9

01:24:32

passed #232509
x86_64-darwin
validate-x86_64-darwin

01:05:27

passed #232513
x86_64-linux
validate-x86_64-linux-deb9-debug

04:42:35

passed #232515
x86_64-linux
validate-x86_64-linux-deb9-dwarf

01:47:23

passed #232514
x86_64-linux
validate-x86_64-linux-deb9-integer-simple

01:34:05

passed #232508
x86_64-linux
validate-x86_64-linux-deb9-unreg-hadrian

02:29:25

passed #232516
x86_64-linux
validate-x86_64-linux-fedora27

00:59:24

failed #232518
x86_64-windows
validate-x86_64-windows

00:39:03

failed #232517
x86_64-windows allowed to fail
validate-x86_64-windows-hadrian

01:24:55

 
  Cleanup
passed #232519
x86_64-darwin
cleanup-darwin

00:00:13

 
  Packaging
skipped #232520
x86_64-linux allowed to fail
doc-tarball
 
  Testing
skipped #232521
x86_64-linux allowed to fail manual
hackage
skipped #232522
x86_64-linux
perf-nofib
 
  Deploy
skipped #232523
x86_64-linux allowed to fail
pages
 
Name Stage Failure
failed
validate-aarch64-linux-deb9 Full Build
Uploading artifacts...
ghc-aarch64-linux-deb9.tar.xz: found 1 matching files
junit.xml: found 1 matching files

performance-metrics.tsv: found 1 matching files

Uploading artifacts to coordinator... ok
id=232510 responseStatus=201 Created token=E9ywjZ2G
Uploading artifacts...
junit.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=232510 responseStatus=201 Created token=E9ywjZ2G
ERROR: Job failed: exit code 1
failed
validate-x86_64-windows-hadrian Full Build
ghc.tar.xz: found 1 matching files                 
WARNING: junit.xml: no matching files
WARNING: Failed to load system CertPool:crypto/x509: system root pool is not available on Windows
Uploading artifacts to coordinator... ok
id=232517 responseStatus=201 Created token=suh4FNnJ
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=12416 revision=1f513601 version=11.10.1
WARNING: junit.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit status 1
failed
validate-x86_64-windows Full Build
Runtime platform                                    arch=amd64 os=windows pid=9984 revision=1f513601 version=11.10.1
WARNING: ghc-x86_64-mingw32.tar.xz: no matching files
WARNING: junit.xml: no matching files
ERROR: No files to upload
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=7660 revision=1f513601 version=11.10.1
WARNING: junit.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit status 2