gdb: fix duplicate test names in gdb.base/break.exp

gdb: fix duplicate test names in gdb.base/break.exp

These test names are duplicated:

      2 PASS: gdb.base/break.exp: set breakpoint via non-integer convenience variable disallowed
      2 PASS: gdb.base/break.exp: set convenience variable $foo to 81.5

Wrap them with `with_test_prefix`. I've actually wrapped a bit more
tests that are related, I think it helps to give the test names a bit
more context.  The modified test names are:

    -PASS: gdb.base/break.exp: set convenience variable $foo to bp_location11
    -PASS: gdb.base/break.exp: set breakpoint via convenience variable
    -PASS: gdb.base/break.exp: set convenience variable $foo to 81.5
    -PASS: gdb.base/break.exp: set breakpoint via non-integer convenience variable disallowed
    +PASS: gdb.base/break.exp: set line breakpoint via convenience variable: set convenience variable $foo to bp_location11
    +PASS: gdb.base/break.exp: set line breakpoint via convenience variable: break $foo
    +PASS: gdb.base/break.exp: set line breakpoint via convenience variable: set convenience variable $foo to 81.5
    +PASS: gdb.base/break.exp: set line breakpoint via convenience variable: non-integer convenience variable disallowed

    -PASS: gdb.base/break.exp: set $l = 47
    -PASS: gdb.base/break.exp: break break.c:$l
    -PASS: gdb.base/break.exp: set convenience variable $foo to 81.5
    -PASS: gdb.base/break.exp: set breakpoint via non-integer convenience variable disallowed
    +PASS: gdb.base/break.exp: set line:file breakpoint via convenience variable: set $l = 47
    +PASS: gdb.base/break.exp: set line:file breakpoint via convenience variable: break break.c:$l
    +PASS: gdb.base/break.exp: set line:file breakpoint via convenience variable: set convenience variable $foo to 81.5
    +PASS: gdb.base/break.exp: set line:file breakpoint via convenience variable: non-integer convenience variable disallowed

gdb/testsuite/ChangeLog:

	* gdb.base/break.exp: Use with_test_prefix.