Fix breakpoint-command.test when no script interpreter is compiled in.

My local build is with -DLLVM_ENABLE_PROJECTS=lldb, but I don't compile
with -DLLDB_ENABLE_PYTHON=True or -DLLDB_ENABLE_LUA=True. This results
in there being no script interpreter.

The test lldb/test/Shell/Breakpoint/breakpoint-command.test has an
implicit dependency on a script interpreter being available.

This patch makes that dependency clear. If you have a script
interpreter, the test gets run, otherwise it gets skipped. This means
that folks (like me) who naively use -DLLVM_ENABLE_PROJECTS=lldb can
continue to run check-all without breakages.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D139463