[gdb/testsuite] Simplify gdb.base/sect-cmd.exp

[gdb/testsuite] Simplify gdb.base/sect-cmd.exp

While looking at gdb.base/sect-cmd.exp, I noticed a few things that can be
simplified:
- use gdb_test instead of gdb_test_multiple
- use -wrap "" as regexp

Also, I noticed this:
...
           fail "$gdb_test_name, saw not found marker"
...
while our usual test naming scheme uses parentheses, like so:
...
           fail "$gdb_test_name (saw not found marker)"
...

Fix the test-name and do the simplifications.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-06-08  Tom de Vries  <tdevries@suse.de>

	* gdb.base/sect-cmd.exp: Use gdb_test.  Use -wrap "".  Fix
	test name.