scripts: Fix the check whether testname is in the only_tests list

When you currently run

  ./run_tests.sh ioapic-split

the kvm-unit-tests run scripts do not only execute the "ioapic-split"
test, but also the "ioapic" test, which is quite surprising. This
happens because we use "grep -w" for checking whether a test should
be run or not.  Because "grep -w" does not consider the "-" character as
part of a word, "ioapic" successfully matches against "ioapic-split".

To fix the issue, use spaces as the only delimiter when running "grep",
removing the problematic "-w" flag from the invocation.  While at it,
add "-F" to avoid unintended use of regular expression metacharacters.

Reported-by: Thomas Huth <thuth@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>