Skip most of FindExt's tests for troublesome configurations.

Skip most of FindExt's tests for troublesome configurations.

There are various various things that break the test's assumptions.
1) If Encode is a static extension, then Configure has special case logic
   to add Encode/* as static extensions
2) -Uusedl causes Encode to be a static extension, and drops building
    XS::APItest and XS::Typemap
3) Any use of -Dnoextensions to choose not to build a extension is not known
   by the test

If any of these are true, FindExt::extensions() and $Config{extensions} will
differ, and most of the tests are going to fail. Moreover, failure doesn't
tell us anything interesting. So don't run those tests.