Fix Meson build system to be able to run tests

[smcv]

Looks like #432 was less complete than I had intended, sorry.

Successfully tested on Debian 10, Ubuntu 20.04, Debian 11 and Debian unstable.

---

• meson.build: Remove unnecessary check for sh

• meson: Build tests with equivalent of -I$(top_srcdir) -I$(top_builddir)

• meson: Build the try-syscall helper

• meson: Run the Python test script with Python, not bash

  The python build option can be used to swap to a different interpreter,
  for environments like the Steam Runtime where the python3 executable in
  the PATH is extremely old but there is a better interpreter available.

  This is treated as non-optional, because Meson is written in Python,
  so the situation where there is no Python interpreter at build-time
  shouldn't arise.

• meson: Make G_TEST_SRCDIR, G_TEST_BUILDDIR match Autotools

• meson: Run test scripts from $srcdir

• tests: Fix compiler warnings for unused arguments

• meson: Actually build and run the tests

• Disable test-specifying-pidns.sh under 'meson dist' while I investigate

  This test is hanging when run under 'meson dist' for some reason, but
  not when run under 'meson test', and not locally, only in the Github
  Workflow-based CI. Disable it for now.

• meson: Improve compatibility with Meson 0.49

  That version doesn't allow more than two arguments for define_variable.

[smcv]

@cgwalters, @alexlarsson or @mwleeds: would you mind taking a look at this? I'd like to do a bubblewrap 0.6.0 release (mainly for #432 and #453, https://github.com/containers/bubblewrap/releases/edit/untagged-a976e728bb68f2b6c8c5), but it seems silly for the first release with a Meson build system to have known bugs for which a solution is available.