Makefile: Remove testdepends from make check dependencies

The new Makefile system, via the helper script in tools/gen_deps.sh, when
generating the targets to test a module, inserts dependencies meaning it
must first check modules this one depends on, whether via 'depends' or
'testdepends' in _info.

Although it seems logical, including 'testdepends' is actually incorrect.
If ccan/a testepends on ccan/b then ccan/b must be *built* in order to test
ccan/a, but it doesn't need to be tested.  testepends are explicitly
permitted to contain loops - it's quite common for two complementary
modules to be used to test each other.  This is one of the reasons
testdepends exists separate from depends.

So, remove testdepends from the generated check dependencies, removing the
circular dependency that Make complains about.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>