quicktest: list SRs only once

The difference is not really measurable.
But I found out there is another pre-existing flag that can speed it up,
`-default-sr`. Then the leak checks are only done on that SR, and not all the
SRs.

So the best way is to run this (at least during development):
```
time ./quicktest.exe -run-only Quicktest_vm_lifecycle -skip-stress -default-sr
[...]
real 0m1.709s
```

Signed-off-by: Edwin Török <edwin.torok@citrix.com>

Author: edwintorok

ocaml/quicktest/qt_filter.ml

@@ -362,9 +362,8 @@ module SR = struct
   let list_srs srs = with_xapi_query srs
 
   let f srs tcs =
-    for_each
-      (fun test_case -> List.map (specialise test_case) (list_srs srs))
-      tcs
+    let srs = list_srs srs in
+    for_each (fun test_case -> List.map (specialise test_case) srs) tcs
 end
 
 let sr = SR.f