Fix conversion tests using windows

Some conversion-related tests are failing under windows:

https://qa.nuxeo.org/jenkins/view/8.x%20-%20master/job/master/job/fullbuild-multidb-windows/lastCompletedBuild/testReport/

org.nuxeo.ecm.core.convert.service.TestConversionService.testReplaceURLsByAbsolutePaths
org.nuxeo.ecm.core.io.download.TestDownloadService.testTransientCleanup
org.nuxeo.ecm.platform.pictures.tiles.service.test.TestService.testTilingSimple
org.nuxeo.ecm.platform.pictures.tiles.service.test.TestService.testTilingSpead
org.nuxeo.ecm.platform.pictures.tiles.service.test.TestService.testLazy
org.nuxeo.ecm.platform.pictures.tiles.service.test.TestService.testTilingWithShrink
org.nuxeo.ecm.platform.pictures.tiles.service.test.TestService.testTilingSimpleMagick
org.nuxeo.ecm.platform.pictures.tiles.service.test.TestService.testMagick
org.nuxeo.ecm.platform.pictures.tiles.service.test.TestService.testMagick2
org.nuxeo.ecm.platform.pictures.tiles.service.test.TestService.testGC
org.nuxeo.ecm.platform.pictures.tiles.service.test.TestService.testBorderTiles
org.nuxeo.ecm.platform.picture.core.test.TestMagickExecutors.testJpegSimplier
org.nuxeo.ecm.platform.picture.core.test.TestMagickExecutors.testCropper
org.nuxeo.ecm.platform.picture.core.test.TestMagickExecutors.testCropperAndResize
org.nuxeo.ecm.platform.picture.core.test.TestMagickExecutors.testConverterWithBmp
org.nuxeo.ecm.platform.picture.core.test.TestMagickExecutors.testConverterWithGif
org.nuxeo.ecm.platform.picture.core.test.TestMagickExecutors.testConverterToPDF
org.nuxeo.ecm.platform.convert.tests.TestOOoServiceManagerService.testServiceRegistration
org.nuxeo.ecm.platform.convert.tests.TestOOoServiceManagerService.testSocketConnection
org.nuxeo.ecm.platform.convert.tests.TestOOoServiceManagerService.testPipeConnection
org.nuxeo.ecm.platform.convert.tests.TestPDFToText.testConverter

Some might be failing because of paths too long, to be checked (see NXP-21615 for org.nuxeo.ecm.core.io.download.TestDownloadService.testTransientCleanup that seem to be independent of conversions)