tests: make .t tests stop immediately if a cd fails

This prevents tests from escaping from TESTTMP as a consequence of a failing
directory creation.

tests/run-tests.py

@@ -659,6 +659,9 b' def tsttest(test, wd, options, replaceme'
             prepos = pos
             pos = n
             addsalt(n, False)
+            cmd = l[4:].split()
+            if len(cmd) == 2 and cmd[0] == 'cd':
+                l = '  $ cd %s || exit 1\n' % cmd[1]
             script.append(l[4:])
         elif l.startswith('  > '): # continuations
             after.setdefault(prepos, []).append(l)