Add an API call to list available solvers for the current platform

[daniel-raffler]

Hello,

this PR extends the JavaSMT API with a new call Solvers.available() to list all solvers that are supported on the current OS and CPU architecture. The call works similarly to the existing Solvers.values() and can be used to iterate over all available solvers during testing. Unlike Solvers.values() it skips over solvers that are not available on the platform, which helps to avoid failed tests due to missing libraries, when in reality the solver just isn't available for the testing platform

The PR also adds a new ant option "ignoreLinkErrors" to decide if tests should fail if solver binaries are not installed, or when there are linking issues due to missing system libraries. By default all SolverBasedTests are skipped if the solver binary can't be found, and only tests in SolverContextFactoryTest will fail as a warning about the configuration issue. Setting the new option "ignoreLinkErrors" to true causes the tests in SolverContextFactoryTest to also be skipped for the solver, while setting it to false makes all tests fail if there are missing binaries or linking issues

An example for how to use the "ignoreLinkErrors" can be found in the updated gitlab-ci.yml: Several of our solvers need a more recent version of libc than is available on the gitlab runner, and we're now setting "ignoreLinkErrors" to true to skip tests that would otherwise fail when these solver can't be loaded