[gdb/testsuite] Remove test-case from test name

[gdb/testsuite] Remove test-case from test name

Remove test-cases from test-names, such that we don't have the redundant:
...
PASS: gdb.base/corefile.exp: backtrace in corefile.exp
...
but simply:
...
PASS: gdb.base/corefile.exp: backtrace
...

Fixed all instances found using:
...
$ grep ":.*:.*\.exp" gdb.sum
...

Tested on x86_64-linux.