gdb/testsuite: move linespec test into gdb.linespec/ directory

The gdb.base/linespecs.exp test should really live in the gdb.linespec
directory, so lets move it there.

As we already have gdb.linespec/linespec.exp, I've renamed the test to
gdb.linespec/errors.exp, as this better reflects what the test is
actually checking.

Finally, the test script doesn't have its own source file, it was
reusing a random other source file, gdb.base/memattr.c.  Now the tests
script is in gdb.linespec/, I've updated the test to use a different
source file from that directory.