Port config tests to littlecheck

This required a bit of thinking.

What we do is we have one test that fakes $HOME, and then we do the
various config tests there.

The fake config we have is reused and we exercise all of the same codepaths.

tests/checks/broken-config.fish

@@ -0,0 +1,20 @@
+#RUN: %fish -C 'set -g fish %fish' %s
+begin
+    set -l dir $PWD/(dirname (status -f))
+    set -gx XDG_CONFIG_HOME $dir/broken-config/
+    set -gx HOME $dir/broken-config/
+    $fish -l -c 'echo but still going'
+    # CHECK: broken
+    # CHECK: but still going
+    # CHECKERR: fish: Unknown command: syntax-error
+    # CHECKERR: ~//fish/config.fish (line {{\d+}}):
+    # CHECKERR: syntax-error
+    # CHECKERR: ^
+    # CHECKERR: from sourcing file ~//fish/config.fish
+    # CHECKERR: called during startup
+
+    $fish -c "echo normal command" -C "echo init"
+    # CHECK: broken
+    # CHECK: init
+    # CHECK: normal command
+end

tests/checks/broken-config/fish/config.fish

@@ -0,0 +1,4 @@
+echo broken
+if status is-login
+    syntax-error
+end