patch 8.1.2131: MSVC tests fail

Problem:    MSVC tests fail.
Solution:   Replace backslashes with slashes.
diff --git a/src/testdir/test_compiler.vim b/src/testdir/test_compiler.vim
index 06522d7..6bb6027 100644
--- a/src/testdir/test_compiler.vim
+++ b/src/testdir/test_compiler.vim
@@ -38,10 +38,11 @@
 endfunc
 
 func Test_compiler_without_arg()
-  let a=split(execute('compiler'))
-  call assert_match($VIMRUNTIME .. '/compiler/ant.vim$',   a[0])
-  call assert_match($VIMRUNTIME .. '/compiler/bcc.vim$',   a[1])
-  call assert_match($VIMRUNTIME .. '/compiler/xmlwf.vim$', a[-1])
+  let runtime = substitute($VIMRUNTIME, '\\', '/', 'g')
+  let a = split(execute('compiler'))
+  call assert_match(runtime .. '/compiler/ant.vim$',   a[0])
+  call assert_match(runtime .. '/compiler/bcc.vim$',   a[1])
+  call assert_match(runtime .. '/compiler/xmlwf.vim$', a[-1])
 endfunc
 
 func Test_compiler_completion()