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run_tests to TestingError so users only ever have one exception to catch.

SyntaxWarning otherwise). So move those imports back to the module level and
save any error messages for when the tests are actually run (if ever).

python -m simtk.testInstallation

runs the tests, whereas

python -c "import simtk.testInstallation"

does not.

Also copy the test PDB file to the data/ directory, since we rather need it...