Improve lldb-mi tests.

summary of changes:
  Cleanup: Use "line_number" API instead of hardcoded source lines
  Combine child.sendline with previous child.send command.
  test_lldbmi_tokens: Add test for MI tokens.
  test_lldbmi_badpathexe: Remove check for prompt so test for bad path can be enabled.

Patch from dawn@burble.org.