test-run-tests: conditionalize the $TESTDIR check for Windows separator

The variable uniformly uses '\' separators, so the straight equality check with
'/' separating the last component fails.  It also doesn't like having the quote
appear in the middle of the string when testing.

tests/test-run-tests.t

@@ -600,7 +600,11 b' running is placed.'
   > - \$TESTDIR, in which test-runtestdir.t is placed (expanded at runtime)
   > - \$RUNTESTDIR, in which run-tests.py is placed (expanded at runtime)
   > 
+  > #if windows
+  >   $ test "\$TESTDIR" = "$TESTTMP\anothertests"
+  > #else
   >   $ test "\$TESTDIR" = "$TESTTMP"/anothertests
+  > #endif
   >   $ test "\$RUNTESTDIR" = "$TESTDIR"
   >   $ head -n 3 "\$RUNTESTDIR"/../contrib/check-code.py
   >   #!/usr/bin/env python