Bug #2240 fix: AsyncCall-related unit tests fail

Bug #2240 fix: AsyncCall-related unit tests fail

Removed libsquid.la from SWAP_TEST_LDADD: Store-related test cases use comm
stubs. They must not include libsquid which provides the true comm code.

Added AsyncCall* sources to selected test cases: Every test case that needs
event.cc also needs AsyncCallQueue and AsyncCall.  Placed all three in
$TEST_CALL_SOURCES variable and used as needed. This prevents test case
compilation failures due to undefined call-related symbols.

I suspect that there are more files in the same "calls" group, but it would be
a lot easier to identify them once we clean src/ up and group files into
subdirs and libraries.