[lldb] Introduce createTestTarget for creating a valid target in API tests
At the moment nearly every test calls something similar to
self.dbg.CreateTarget(self.getBuildArtifact("a.out")) and them sometimes
checks if the created target is actually valid with something like
self.assertTrue(target.IsValid(), "some useless text").
Beside being really verbose the error messages generated by this pattern are
always just indicating that the target failed to be created but now why.
This patch introduces a helper function createTestTarget to our Test class
that creates the target with the much more verbose CreateTarget overload that
gives us back an SBError (with a fancy error). If the target couldn't be created
the function prints out the SBError that LLDB returned and asserts for us. It
also defaults to the "a.out" build artifact path that nearly all tests are using
to avoid to hardcode "a.out" in every test.
I converted a bunch of tests to the new function but I'll do the rest of the
test suite as follow ups.
Reviewed By: JDevlieghere
Differential Revision: https://reviews.llvm.org/D102771