Re-add explicit littlecheck HOME/XDG_CONFIG_HOME overrides on request

This was requested by a team member who would like for some tests to
remain invokable (in thier own $HOME) directly via littlecheck without
relying on the test driver to prep the environment.

A comment explaining the rationale is also added so this doesn't get
passed down as folklore "you need to include this for tests to run" even
though no one understands why.

tests/checks/features-qmark1.fish

@@ -1,2 +1,3 @@
-#RUN: env %fish --features '' -c 'string match --quiet "??" ab ; echo "qmarkon: $status"'
+# Explicitly overriding HOME/XDG_CONFIG_HOME is only required if not invoking via `make test`
+# RUN: env HOME="$(mktemp -d)" XDG_CONFIG_HOME="$(mktemp -d)" %fish --features '' -c 'string match --quiet "??" ab ; echo "qmarkon: $status"'
 #CHECK: qmarkon: 0

tests/checks/set.fish

@@ -1,4 +1,5 @@
-# RUN: env FISH=%fish %fish %s
+# Explicitly overriding HOME/XDG_CONFIG_HOME is only required if not invoking via `make test`
+# RUN: env FISH=%fish HOME="$(mktemp -d)" XDG_CONFIG_HOME="$(mktemp -d)" %fish %s
 # Environment variable tests
 
 # Test if variables can be properly set
@@ -802,7 +803,7 @@ function test-function-scope
     #CHECK: $globalvar: set in global scope, unexported, with 1 elements
     #CHECK: $globalvar[1]: |global|
 end
-        
+
 test-function-scope
 echo $funcvar $funcvar2
 # CHECK:

tests/checks/symlinks-not-overwritten.fish

@@ -1,4 +1,5 @@
-#RUN: %fish -C 'set -g fish %fish' %s
+# Explicitly overriding HOME/XDG_CONFIG_HOME is only required if not invoking via `make test`
+# RUN: env HOME="$(mktemp -d)" XDG_CONFIG_HOME="$(mktemp -d)" %fish -C 'set -g fish %fish' %s
 
 mkdir -p $XDG_CONFIG_HOME/fish