[gdb/testsuite] Fix double debug info in gdb.dwarf2/dw2-ref-missing-frame.exp

[gdb/testsuite] Fix double debug info in gdb.dwarf2/dw2-ref-missing-frame.exp

A mistake slipped in in commit a5ea23036d8 "[gdb/testsuite] Use function_range
in gdb.dwarf2/dw2-ref-missing-frame.exp".

Before the commit the main file was compiled with debug info, and the two
others not:
...
if {[prepare_for_testing_full "failed to prepare" \
        [list $testfile {} $srcfile {} $srcfuncfile {} \
             $srcmainfile debug]]} {
...

After the commit, all were compiled with debug info, and consequently, there
are two versions of debug info for $srcfuncfile.  This shows up as a FAIL when
running the test-case with target boards readnow and cc-with-debug-names.

Fix this by using prepare_for_testing_full, as before.

Tested on x86_64-linux.

Fixes: a5ea23036d8 ("[gdb/testsuite] Use function_range in
       gdb.dwarf2/dw2-ref-missing-frame.exp")