gdb/testsuite: introduce gdb_step_until

gdb/testsuite: introduce gdb_step_until

Currently, GDB's testsuite uses a set amount of step commands to exit
functions. This is a problem if a compiler emits different epilogue
information from gcc, or emits no epilogue information at all. It was
most noticeable if Clang was used to test GDB.

To fix this unreliability, this commit introduces a new proc that will
step the inferior until it is stopped at a line that matches the given
regexp, or until it steps too many times - defined as an optional
argument. If the line is found, it shows up as a single PASS in the
test, and if the line is not found, a single FAIL is emitted.

This patch only introduces this proc, but does not add it to any
existing tests, these will be introduced in the following commit.