meson: Fix test failures on the CI around nunit-runner

environment().prepend() does not stack. None of the operations on that
object stack. This is documented in the Meson reference manual. Also
avoid overriding previously-set operations because that will cause
a warning in a future version of Meson.

Also, we do not need to manually construct MONO_PATH values. We can
pass an array and have Meson construct it for us.

Tests/meson.build

@@ -1,6 +1,10 @@
 nunit_console = find_program('nunit-console', 'nunitlite-runner', required: get_option('tests'))
 
 if nunit_console.found()
+    nunit_tests_env = environment()
+    # FIXME: port this to macOS and Windows
+    nunit_tests_env.prepend('LD_LIBRARY_PATH', testsenv_ld_library_path)
+
     nunit_version = '3.10.1'
     get_nunit_res = run_command(nuget, 'get',
         '--builddir=NUnit',
@@ -12,20 +16,27 @@ if nunit_console.found()
     )
 
 
+    nunit_mono_path = []
     nunit_dep = dependency('mono-nunit', required: false, version: ['>=2.6', '< 2.7'])
     if not nunit_dep.found()
         if get_nunit_res.returncode() != 0
             message('Failed to get NUnit: ' + get_nunit_res.stderr())
         else
             foreach path: get_nunit_res.stdout().split()
-                testsenv.prepend('MONO_PATH',
+              nunit_mono_path += [join_paths(meson.build_root(), path.strip('-r:'), '..')]
-                    join_paths(meson.build_root(), path.strip('-r:'), '..'))
             endforeach
 
             nunit_dep = declare_dependency(link_args: get_nunit_res.stdout().split(),
                 version: nunit_version)
         endif
     endif
+
+    if nunit_mono_path.length() > 0
+        nunit_tests_env.prepend('MONO_PATH', nunit_mono_path + mono_path)
+    else
+        nunit_tests_env.prepend('MONO_PATH', mono_path)
+    endif
+
     if nunit_dep.found()
         foreach test: [
                 # 'PipelineTests',
@@ -34,7 +45,7 @@ if nunit_console.found()
             lib = shared_library(test, test + '.cs', 'TestBase.cs',
                         cs_args: ['-nowarn:169', '-nowarn:108', '-nowarn:114'],
                         dependencies: [gst_sharp_dep, nunit_dep])
-            test(test, nunit_console, args: [lib.full_path()], env: testsenv)
+            test(test, nunit_console, args: [lib.full_path()], env: nunit_tests_env)
         endforeach
     endif
 else

meson.build

@@ -41,13 +41,7 @@ else
       required: false)
 endif
 
-if host_machine.system() == 'windows'
+mono_path = [gtk_sharp.get_variable('mono_path'), join_paths(meson.current_build_dir(), 'sources')]
-    pathsep = ';'
-else
-    pathsep = ':'
-endif
-mono_path = gtk_sharp.get_variable('mono_path') + pathsep
-mono_path += pathsep + join_paths(meson.current_build_dir(), 'sources')
 
 codegen_dependencies = [gapi_codegen, gapi_fixup, glib_sharp, gio_sharp]
 gapi_fixup = gapi_fixup.full_path()
@@ -95,15 +89,16 @@ if ges_dep.found()
     gst_deps_defs +=  [
         ['gst-editing-services', ['gst-editing-services', 'ges_dep'], 'libges'],
     ]
-    mono_path += pathsep + join_paths(meson.current_build_dir(), 'ges')
+    mono_path += [join_paths(meson.current_build_dir(), 'ges')]
 endif
 
 testsenv = environment()
 testsenv.prepend('MONO_PATH', mono_path)
 i = 0
+testsenv_ld_library_path = []
 foreach dep: gst_deps + [ges_dep]
     if dep.type_name() == 'pkgconfig'
-        testsenv.prepend('LD_LIBRARY_PATH', dep.get_pkgconfig_variable('libdir'))
+        testsenv_ld_library_path += [dep.get_pkgconfig_variable('libdir')]
     else
         depdef = gst_deps_defs[i][1]
         libname = gst_deps_defs[i].get(2, '')
@@ -118,11 +113,13 @@ foreach dep: gst_deps + [ges_dep]
                 endif
                 j += 1
             endforeach
-            testsenv.prepend('LD_LIBRARY_PATH', dirname)
+            testsenv_ld_library_path += [dirname]
         endif
     endif
     i += 1
 endforeach
+# FIXME: port this to macOS and Windows
+testsenv.prepend('LD_LIBRARY_PATH', testsenv_ld_library_path)
 
 subdir('sources')
 if ges_dep.found()