cleanup TEST_EXES in src/CMakeLists.txt

[cwsmith]

Calls like this:

omega_h/src/CMakeLists.txt

Line 389 in 6216b3a

set(TEST_EXES ${TEST_EXES} unit_math)

should be added to the macro that adds tests.

TEST_EXES is used here:

omega_h/src/CMakeLists.txt

Lines 548 to 549 in 6216b3a

	add_custom_target(check COMMAND ${CMAKE_CTEST_COMMAND}
	DEPENDS ${TEST_EXES})