gdb/testsuite: Remove duplicates from gdb.mi/mi-catch-load.exp

When I run the testsuite, I have:

    Running .../gdb/testsuite/gdb.mi/mi-catch-load.exp ...
    DUPLICATE: gdb.mi/mi-catch-load.exp: breakpoint at main
    DUPLICATE: gdb.mi/mi-catch-load.exp: mi runto main

Fix by grouping the various phases in with_test_prefix blocks.  Since
the tests now have a prefix, remove the manually written prefixes in
testnames.

Also change some messages with the pattern "(timeout) $testname" into
"$estname (timeout)" since tools will handle this as $testname[1] (which
is what we want in this particular scenario).

Tested on x86_64-linux.

[1] https://sourceware.org/gdb/wiki/GDBTestcaseCookbook#Do_not_use_.22tail_parentheses.22_on_test_messages