GetSortedAreas safety and area unit test fixes (#22851)

Fixes a helper proc that, among other things, had quietly broken all our
area building unit tests.

Fixes every single goddamn area that had been mapped wrong because the
unit tests weren't running.

changes:
- code_imp: "Updates map_path var for mapping unit tests to be a list
instead of a var, allowing for opting-in of unit tests on by-map basis."
- code_imp: "Fire alarm directionality map test updated for (almost) all
directional wall-mounted objects, with only sccv_horizon map currently
opted-in."
- bugfix: "Adds extra safety for GLOB.SortedAreas having entries removed
but not later re-generated."
- bugfix: "Fixes all the incorrectly mapped areas on the Horizon exposed
by newly-run unit tests."
- imageadd: "Adds currently unused map_helper sprites for airlocks- prep
for future work."

code/unit_tests/ss_test.dm

@@ -165,7 +165,7 @@ SUBSYSTEM_DEF(unit_tests)
 
 		TEST_GROUP_OPEN("[test.name]")
 
-		if (test.map_path && SSatlas.current_map && SSatlas.current_map.path != test.map_path)
+		if (length(test.map_path) && SSatlas.current_map && !(SSatlas.current_map.path in test.map_path))
 			test.pass(TEST_OUTPUT_GREEN("Check Disabled: This test is not allowed to run on this map."), __FILE__, __LINE__)
 			TEST_GROUP_CLOSE("[test.name]")
 			if (MC_TICK_CHECK)