cgo: do a basic test that math functions work

They should, but we weren't testing this.
I discovered this while working on
https://github.com/tinygo-org/macos-minimal-sdk/pull/4 which will likely
make math.h not work anymore. So I wanted to make sure we have a test in
place before we update that dependency.

testdata/cgo/out.txt

@@ -74,4 +74,6 @@ C.GoString(nil):
 len(C.GoStringN(nil, 0)): 0
 len(C.GoBytes(nil, 0)): 0
 copied string: foobar
+CGo sqrt(3): +1.732051e+000
+C   sqrt(3): +1.732051e+000
 line written using C puts