gdb/testsuite: fix "set temporary breakpoint" DUPLICATEs

Commit c67f4e53 ("gdb/testsuite: make gdb.ada/mi_prot.exp stop at
expected location") introduced some DUPLICATEs in MI tests using
mi_continue_to_line, for example:

    DUPLICATE: gdb.ada/mi_ref_changeable.exp: mi_continue_to_line: set temporary breakpoint

These test names were previously differentiated by the location passed
to mi_continue_to_line.  Since the location can contain a path, that
commit removed the location from the test name, in favor of a hardcoded
string "set temporary breakpoint", hence removing the differentiator.

mi_continue_to_line receives a "test" parameter, containing a test
name.  Add a "with_test_prefix" with that name, so that all tests
recorded during mi_continue_to_line have this in their name.

mi_continue_to_line passes that "test" string to mi_get_stop_line, that
is a bit superfluous.  mi_get_stop_line only uses that string in case of
failures (it doesn't record a pass if everything goes fine).  Since it's
not crucial, just remove it, and adjust all callers.

Adjust three gdb.mi/mi-var-*.exp tests to use prefixes to differentiate
the multiple calls to mi_run_inline_test (which calls
mi_continue_to_line).

Change-Id: I511c6caa70499f8657b1cde37d71068d74d56a74