LLInstanceTracker<T> performs validation in ~LLInstanceTracker(). Normally
validation failure logs an error and terminates the program, which is fine. In
the test executable, though, we want validation failure to throw an exception
instead so we can catch it and continue testing other failure conditions. But
since destructors in C++11 are implicitly noexcept(true), that exception never
made it out of ~LLInstanceTracker(): it crashed the test program instead.
Declaring ~LLInstanceTracker() noexcept(false) solves that, allowing the test
program to catch the exception and continue.

However, if we unconditionally declare that, then every destructor anywhere in
the inheritance hierarchy for any LLInstanceTracker subclass must also be
noexcept(false)! That's way too pervasive, especially for functionality we
only need (or want) in a specific test executable.

Instead, make the CMake macros LL_ADD_PROJECT_UNIT_TESTS() and
LL_ADD_INTEGRATION_TEST() -- with which we define all viewer build-time tests
-- define two new command-line macros: LL_TEST=testname and LL_TEST_testname.
That way, preprocessor logic in a header file can detect whether it's being
compiled for production code or for a test executable.

(While at it, encapsulate in a new GET_OPT_SOURCE_FILE_PROPERTY() CMake macro
an ugly repetitive pattern. The builtin GET_SOURCE_FILE_PROPERTY() sets the
target variable to "NOTFOUND" -- rather than an empty string -- if the
specified property wasn't set. Every call to GET_SOURCE_FILE_PROPERTY() in
LL_ADD_PROJECT_UNIT_TESTS() was followed by a test for NOTFOUND and an
assignment to "". Wrap all that in a macro whose 'unset' value is "".)

Now llinstancetracker.h can detect when we're building the LLInstanceTracker
unit test executable, and *only then* declare ~LLInstanceTracker() as
noexcept(false). We #define LLINSTANCETRACKER_DTOR_NOEXCEPT to expand either
empty or noexcept(false), also detecting clang in C++11 mode. (It all works
fine without noexcept(false) until we turn on C++11 mode.)

We also use that macro for the StatBase class in lltrace.h. Turns out some of
the infrastructure headers required for tests in general, including the
LLInstanceTracker test, use LLInstanceTracker. Fortunately that appears to be
the only other class we must annotate this way for the LLInstanceTracker tests.