Revert "build/meson: name platform tests like autotools"

Older versions of meson don't support building the same names
multiple times.

  Meson encountered an error in file src/tests/meson.build, line 14, column 2:
  Tried to create target "test-general", but a target of that name already exists.

We really need to use unique filenames everywhere. Revert the name
change for now.

This breaks again the valgrind workaround in "tools/run-nm-test.sh".

This reverts commit 5466edc63e.

src/platform/tests/meson.build

@@ -13,13 +13,13 @@ test_units = [
 
 foreach test_unit: test_units
   exe = executable(
-    test_unit[0],
+    'platform-' + test_unit[0],
     test_unit[1],
     dependencies: test_unit[2],
   )
 
   test(
-    test_unit[0],
+    'platform/' + test_unit[0],
     test_script,
     timeout: test_unit[3],
     args: test_args + [exe.full_path()],