gdb/testsuite: Make test names unique in gdb.linespec tests

Make test names unique in the gdb.linespec tests.  On my local machine
this removed 43 duplicate test names.  It is possible that different
setups might still encounter some duplicates.

gdb/testsuite/ChangeLog:

	* gdb.linespec/explicit.exp: Make test names unique.
	* gdb.linespec/ls-errs.exp: Likewise.