[gdb/testsuite] Skip dap tests for tcl 8.5

[gdb/testsuite] Skip dap tests for tcl 8.5

When running the dap tests on a system with tcl 8.5, we run into:
...
ERROR: tcl error sourcing gdb/testsuite/gdb.dap/memory.exp.
ERROR: bad class "entier": must be alnum, alpha, ascii, control, boolean, \
  digit, double, false, graph, integer, list, lower, print, punct, space, \
  true, upper, wideinteger, wordchar, or xdigit
    while executing
"string is entier $num"
    (procedure "num" line 16)
    invoked from within
...

Fix this by:
- requiring tcl 8.6 in allow_dap_tests, and
- adding the missing require allow_dap_tests in gdb.dap/memory.exp.

Tested on x86_64-linux.