meson: Do not use path separator in test names

Avoiding warnings like:

    WARNING: Target "elements/audioamplify" has a path separator in its name.

tests/check/meson.build

@@ -157,7 +157,8 @@ test_deps = [gst_dep, gstbase_dep, gstnet_dep, gstcheck_dep, gstaudio_dep,
 
 # FIXME: add valgrind suppression common/gst.supp gst-plugins-good.supp
 foreach t : good_tests
-  test_name = t.get(0)
+  fname = '@0@.c'.format(t.get(0))
+  test_name = t.get(0).underscorify()
   extra_deps = [ ]
   if t.length() == 3
     extra_deps = t.get(2)
@@ -178,7 +179,7 @@ foreach t : good_tests
     env.set('GSETTINGS_BACKEND', 'memory')
 
     env.set('GST_REGISTRY', '@0@/@1@.registry'.format(meson.current_build_dir(), test_name))
-    exe = executable(test_name, '@0@.c'.format(test_name),
+    exe = executable(test_name, fname,
       include_directories : [configinc],
       c_args : ['-DHAVE_CONFIG_H=1' ] + test_defines,
       dependencies : [libm] + test_deps + extra_deps,