meson: stop registering .standalone in executables_by_name

meson: stop registering .standalone in executables_by_name

af4c5730e524c972994232259e5051b951142485 tried to have meson stop
building all possible .standalone binaries in for tests. But it turns out
that those binaries are still built for meson-test-prereq, because they
are listed in executables_by_name, and we interate over
executables_by_name and add all exes found there as dependencies for two
tests: test-link-abi and libshared-unused-symbols. So we'd end up still
building all the .standalone binaries. To really fix this, define the
executables() for .standalone targets, but don't add them to this array.

A secondary change is to set build_by_default to true for .standalone
binaries if have_dlopen_tests is set. This means that we'll build more
binaries in the "build" phase, instead of only building them "on demand"
for tests. I think it is nicer to build everything that'll be used in one
step, and then only run the tests in the test target. For example, tests
may be run under root or in some special environment, and building thins
there is iffy.

The overall effect of this change should be that we stop building or
testing .standalone variants for binaries that we'll not later install,
unless -Ddlopen-tests=true is specified.

Also exclude .standalone binaries from check-help test. The test fails
for some binaries when the width is exceeded because of the ".standalone"
suffix in the name. The actual binary would be called without the
suffix, so the test is not testing a realistic scenario.