You are close - the test property is called LABELS, not LABEL.
There are several ways to set tags; the one you selected (using set_tests_properties) has a slight error. Signature:
set_tests_properties(test1 [test2...] PROPERTIES prop1 value1 prop2 value2)
, . , , "" CMake, , :
set_tests_properties(FirstTest PROPERTIES LABELS "TESTLABEL;UnitTest;FooModule")
set(Labels TESTLABEL UnitTest FooModule)
set_tests_properties(FirstTest PROPERTIES LABELS "${Labels}")
, , set_property:
set_property(TEST FirstTest PROPERTY LABELS TESTLABEL UnitTest FooModule)
set_property(TEST FirstTest PROPERTY LABELS ${Labels})
, .