harden readers.stac tests

[hobu]

Let's just ignore the test if the schema_validate_test can't access the schemas. It's also possible that schemas.stacspec.org has gone away.

2024-04-10T14:59:14.8005900Z 51: unknown file: Failure
2024-04-10T14:59:14.8012760Z 51: C++ exception with description "Could not read from schemas.stacspec.org/v1.0.0/item-spec/json-schema/item.json" thrown in the test body.
2024-04-10T14:59:14.8016800Z 51: [  FAILED  ] StacReaderTest.wrench_test (255975 ms)
2024-04-10T14:59:14.8018890Z 51: [ RUN      ] StacReaderTest.schema_validate_test
2024-04-10T15:03:30.6331790Z 51: unknown file: Failure
2024-04-10T15:03:30.6337310Z 51: C++ exception with description "Could not read from schemas.stacspec.org/v1.0.0/catalog-spec/json-schema/catalog.json" thrown in the test body.
2024-04-10T15:03:30.6342700Z 51: [  FAILED  ] StacReaderTest.schema_validate_test (255833 ms)
2024-04-10T15:03:30.6345900Z 51: [----------] 18 tests from StacReaderTest (529653 ms total)
2024-04-10T15:03:30.6348340Z 51: 
2024-04-10T15:03:30.6350310Z 51: [----------] Global test environment tear-down
2024-04-10T15:03:30.6352990Z 51: [==========] 18 tests from 1 test suite ran. (529653 ms total)
2024-04-10T15:03:30.6355140Z 51: [  PASSED  ] 16 tests.
2024-04-10T15:03:30.6356770Z 51: [  FAILED  ] 2 tests, listed below:
2024-04-10T15:03:30.6358890Z 51: [  FAILED  ] StacReaderTest.wrench_test
2024-04-10T15:03:30.6361290Z 51: [  FAILED  ] StacReaderTest.schema_validate_test
2024-04-10T15:03:30.6363350Z 51: 
2024-04-10T15:03:30.6364670Z 51:  2 FAILED TESTS
2024-04-10T15:03:30.6645000Z  51/147 Test  #51: pdal_io_stac_reader_test .....................***Failed  529.71 sec
2024-04-10T15:03:30.6653260Z test 52

[hobu]

We think this was #4404.