Fix llprocess_test.cpp's exception catching for Linux.

In the course of re-enabling the indra/test tests last year, Log generalized a
workaround I'd introduced in llsdmessage_test.cpp. In Linux viewer land, a
test program trying to catch an expected exception can't seem to catch it by
its specific class (across the libllcommon.so boundary), but must instead
catch std::runtime_error and validate the typeid().name() string. Log added a
macro for this idiom in llevents_tut.cpp. Generalize that macro further for
normal-case processing as well, move it to a header file of its own and use it
in all known places -- plus the new exception-catching tests in
llprocess_test.cpp.