Avoid building the tests again for example project with add_subdirectory #94

Tests ✅

Report	Passed	Time
tests/convert/result-junit-linux.xml	1✅	93ms
tests/convert/result-junit-mac.xml	1✅	126ms
tests/convert/result-junit-msvc.xml	1✅	178ms
tests/core/result-junit-linux.xml	1✅	1ms
tests/core/result-junit-mac.xml	1✅	2ms
tests/core/result-junit-msvc.xml	1✅	2ms

1 tests were completed in 93ms with 1 passed, 0 failed and 0 skipped.

Test suite	Passed	Failed	Skipped	Time
convert_tests	1✅			93ms

convert_tests.global
  ✅ Convert from ascii to binary

1 tests were completed in 126ms with 1 passed, 0 failed and 0 skipped.

Test suite	Passed	Failed	Skipped	Time
convert_tests	1✅			126ms

convert_tests.global
  ✅ Convert from ascii to binary

1 tests were completed in 178ms with 1 passed, 0 failed and 0 skipped.

Test suite	Passed	Failed	Skipped	Time
convert_tests.exe	1✅			178ms

convert_tests.exe.global
  ✅ Convert from ascii to binary

1 tests were completed in 1ms with 1 passed, 0 failed and 0 skipped.

Test suite	Passed	Failed	Skipped	Time
core_tests	1✅			1ms

core_tests.global
  ✅ Search for GCode blocks

1 tests were completed in 2ms with 1 passed, 0 failed and 0 skipped.

Test suite	Passed	Failed	Skipped	Time
core_tests	1✅			2ms

core_tests.global
  ✅ Search for GCode blocks

1 tests were completed in 2ms with 1 passed, 0 failed and 0 skipped.

Test suite	Passed	Failed	Skipped	Time
core_tests.exe	1✅			2ms

core_tests.exe.global
  ✅ Search for GCode blocks