Add testTarget for SwiftPM

[cntrump]

Run swift test case:

$ swift test

Output:

[157/157] Build complete!
Test Suite 'All tests' started at 2021-12-01 22:44:10.592
Test Suite 'tommathPackageTests.xctest' started at 2021-12-01 22:44:10.593
Test Suite 'LibTommathTests' started at 2021-12-01 22:44:10.593
Test Case '-[TommathTests.LibTommathTests testTrivialStuff]' started.
Test Case '-[TommathTests.LibTommathTests testTrivialStuff]' passed (0.001 seconds).
Test Suite 'LibTommathTests' passed at 2021-12-01 22:44:10.594.
	 Executed 1 test, with 0 failures (0 unexpected) in 0.001 (0.001) seconds
Test Suite 'tommathPackageTests.xctest' passed at 2021-12-01 22:44:10.594.
	 Executed 1 test, with 0 failures (0 unexpected) in 0.001 (0.001) seconds
Test Suite 'All tests' passed at 2021-12-01 22:44:10.594.
	 Executed 1 test, with 0 failures (0 unexpected) in 0.001 (0.002) seconds