tests: Remove the need to pass $FISH_SOURCE_DIR

This is only used in check-translations, where we can just use a
relative path from the test script

cmake/Tests.cmake

@@ -40,7 +40,6 @@ set(SKIP_RETURN_CODE 125)
 # The top-level test target is "fish_run_tests".
 add_custom_target(fish_run_tests
   COMMAND env CTEST_PARALLEL_LEVEL=${CTEST_PARALLEL_LEVEL} FISH_FORCE_COLOR=1
-          FISH_SOURCE_DIR=${CMAKE_SOURCE_DIR}
           ${CMAKE_CTEST_COMMAND} --force-new-ctest-process # --verbose
           --output-on-failure --progress
   DEPENDS fish fish_indent fish_key_reader fish_test_helper

tests/checks/check-translations.fish

@@ -1,8 +1,10 @@
 #RUN: fish=%fish %fish %s
 #REQUIRES: msgfmt --help
 
+set -l dir (status dirname)
+
 set -l fail_count 0
-for file in $FISH_SOURCE_DIR/po/*.po
+for file in $dir/../../po/*.po
     # We only check the format strings.
     # Later on we might do a full "--check" to also check the headers.
     msgfmt --check-format $file