Eliminate shared temporary directory for tests

Aside from the fact that the shared state could cause problems, tests
were randomly assuming it would be created where that wasn't the case.
In particular, `redirect.fish` and `basic.fish` were failing on only
macOS because `../test/temp` didn't exist yet - it would be created by
other tests later.

tests/checks/basic.fish

@@ -42,12 +42,12 @@ end
 # Simple function tests
 
 function foo
-    echo >../test/temp/fish_foo.txt $argv
+    echo >$TMPDIR/fish_foo.txt $argv
 end
 
 foo hello
 
-cat ../test/temp/fish_foo.txt |read foo
+cat $TMPDIR/fish_foo.txt |read foo
 
 if test $foo = hello;
   echo Test 2 pass
@@ -297,7 +297,7 @@ else if test -n "def"
 else if not_a_valid_command but it should be OK because a previous branch was taken
 	echo "epsilon 5.3"
 else if test ! -n "abc"
-	echo "epsilon 5.4"	
+	echo "epsilon 5.4"
 end
 #CHECK: epsilon5.2
 
@@ -327,10 +327,10 @@ type -q -f fish_test_type_zzz ; echo $status
 
 # ensure that builtins that produce no output can still truncate files
 # (bug PCA almost reintroduced!)
-echo abc > ../test/temp/file_truncation_test.txt
-cat ../test/temp/file_truncation_test.txt
-echo -n > ../test/temp/file_truncation_test.txt
-cat ../test/temp/file_truncation_test.txt
+echo abc > $TMPDIR/file_truncation_test.txt
+cat $TMPDIR/file_truncation_test.txt
+echo -n > $TMPDIR/file_truncation_test.txt
+cat $TMPDIR/file_truncation_test.txt
 #CHECK: abc
 
 # Test events.