[gdb/testsuite] Fix timeout in gdb.threads/detach-step-over.exp with readnow

When running test-case gdb.threads/detach-step-over.exp with target board
readnow, I run into:
...
Reading symbols from /lib64/libc.so.6...^M
Reading symbols from \
  /usr/lib/debug/lib64/libc-2.26.so-2.26-lp152.26.6.1.x86_64.debug...^M
Expanding full symbols from \
  /usr/lib/debug/lib64/libc-2.26.so-2.26-lp152.26.6.1.x86_64.debug...^M
FAIL: gdb.threads/detach-step-over.exp: \
  breakpoint-condition-evaluation=host: target-non-stop=on: non-stop=on: \
  displaced=off: iter 2: attach (timeout)
...

Fix this by doing exp_continue when encountering the "Reading symbols" or
"Expanding full symbols" lines.

This is still fragile and times out with a higher load, similated f.i. by
stress -c 5.  Fix that by using a timeout factor of 2.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-05-05  Tom de Vries  <tdevries@suse.de>

	* gdb.threads/detach-step-over.exp: Do exp_continue when encountering
	"Reading symbols" or "Expanding full symbols" lines.  Using timeout
	factor of 2 for attach.