[gdb/testsuite] Give up after consecutive timeouts in completion-support.exp

When running test-case gdb.linespec/cpcompletion.exp with target board
unix/-flto/-O0/-flto-partition=none/-ffat-lto-objects, we run into lots of
timeouts, in particular with this pattern:
...
FAIL: gdb.linespec/cpcompletion.exp: template-ret-type: \
  cmd complete "b template2_"
FAIL: gdb.linespec/cpcompletion.exp: template-ret-type: \
  tab complete "b template2_st" (timeout)
FAIL: gdb.linespec/cpcompletion.exp: template-ret-type: \
  cmd complete "b template2_st"
FAIL: gdb.linespec/cpcompletion.exp: template-ret-type: \
  tab complete "b template2_str" (timeout)
FAIL: gdb.linespec/cpcompletion.exp: template-ret-type: \
  cmd complete "b template2_str"
FAIL: gdb.linespec/cpcompletion.exp: template-ret-type: \
  tab complete "b template2_stru" (timeout)
...

Fix this by detecting timeouts in test_complete_prefix_range_re and giving up
after 3 consecutive timeouts.

This reduces testing time from ~39m to ~9m.

Tested on x86_64-linux.