[gdb/testsuite] Skip gdb.base/readnever.exp with target board readnow

[gdb/testsuite] Skip gdb.base/readnever.exp with target board readnow

When running test-case gdb.base/readnever.exp with target board readnow, we
have:
...
spawn gdb -nw -nx -data-directory data-directory -ex set sysroot -readnow \
  --readnever^M
gdb: '--readnow' and '--readnever' cannot be specified simultaneously^M
ERROR: : spawn id exp19 not open
...

Fix this by skipping the test when -readnow/--readnow is detected in
GDBFLAGS.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-04-23  Tom de Vries  <tdevries@suse.de>

	* gdb.base/readnever.exp: Skip if GDBFLAGS contain -readnow/--readnow.