HANG nested signal lost

The new signest test that I added has passed every time except just now it
failed (can't repro manually). It looks like a lost nested signal leading
to a hang:

```
debug-internal-32: 224 tests passed, **** 1 tests failed: ****
code_api|linux.signest
```

```
68/225 Testing: code_api|linux.signest
68/225 Test: code_api|linux.signest
Command: "/work/dr/build_suite/build_debug-internal-32/bin32/drrun" "-s" "60" "-quiet" "-debug" "-killpg" "-dr_home" "/work/dr/build_suite/install" "-stderr_mask" "0xC" "-dumpcore_mask" "0" "-code_api" "--" "/work/dr/build_suite/build_debug-internal-32/suite/tests/bin/linux.signest"
Directory: /work/dr/build_suite/build_debug-internal-32/suite/tests
"code_api|linux.signest" start time: Aug 05 20:02 EDT
Output:
----------------------------------------------------------
sending 2 signals
in handler 42

Test time = 60.00 sec
----------------------------------------------------------
Test Fail Reason:
Required regular expression not found.Regex=[^sending 2 signals
in handler 42
in handler 42
all done
$
]
"code_api|linux.signest" end time: Aug 05 20:03 EDT
"code_api|linux.signest" time elapsed: 00:01:00
----------------------------------------------------------
```