[lldb] [test] Workaround symlink-related test failures

Use realpath() when spawning the executable create_after_attach
to workaround a FreeBSD plugin (and possibly others) problem.
If the executable is started via a path containing a symlink, it is
added to the module list twice -- via the real and apparent path.
This in turn cases the requested breakpoint to resolve twice.

Use realpath() for main program path in lldb-vscode breakpoint tests
to workaround a similar problem.  If the passed path does not match
the realpath, lldb-vscode does not report the breakpoints as verified
and causes tests to fail.

Since the underlying problems are non-trivial to fix and the purpose
of these tests is not to reproduce symlink problems, let's apply
trivial workarounds to make them pass.

Differential Revision: https://reviews.llvm.org/D97230