meson: add tests (#1044)

This integrates the "main" test suite (test/test.cc) in Meson.

This allows to run the tests in the CI with the Meson-built version of
the library to ensure that nothing breaks unexpectedly.

It also simplifies life of downstream packagers, that do not have to
write a custom build script to split the library and run tests but can
instead just let Meson do that for them.

test/www/dir/meson.build

@@ -0,0 +1,7 @@
+# SPDX-FileCopyrightText: 2021 Andrea Pappacoda
+#
+# SPDX-License-Identifier: MIT
+
+configure_file(input: 'index.html', output: 'index.html', copy: true)
+configure_file(input: 'test.abcde', output: 'test.abcde', copy: true)
+configure_file(input: 'test.html',  output: 'test.html',  copy: true)