Cygwin: Makefile: Drop all the "test dll" considerations

Cygwin: Makefile: Drop all the "test dll" considerations

After 90236c3a2cf6, the testsuite is failing, as the cygwin0.dll
referenced by the implib that testsuite programs are linked with doesn't
exist anymore.

We don't need to make and link the testsuite with a specially named DLL,
as the cygwin DLL (since 526b0fbca377) takes into consideration the path
it's executing from to define separate "Cygwin installations", which
don't interact.

Fixes: 90236c3a2cf6 ("Cygwin: Makefile: build new-cygwin1.dll in a single step")