gdb/testsuite: fix "continue outside of loop" TCL errors

gdb/testsuite: fix "continue outside of loop" TCL errors

Many test cases had a few lines in the beginning that look like:

if { condition } {
  continue
}

Where conditions varied, but were mostly in the form of ![runto_main] or
[skip_*_tests], making it quite clear that this code block was supposed
to finish the test if it entered the code block. This generates TCL
errors, as most of these tests are not inside loops.  All cases on which
this was an obvious mistake are changed in this patch.