tests: use an alias to make msys 'pwd' return paths with forward slashes

tests/run-tests.py

@@ -593,6 +593,8 b' def tsttest(test, wd, options, replaceme'
     f.close()
 
     script = []
+    if os.getenv('MSYSTEM'):
+        script.append('alias pwd="pwd -W"\n')
     for n, l in enumerate(t):
         if not l.endswith('\n'):
             l += '\n'