use BATS_FILE_TMPDIR in test/plugin test

use global setup with bats with possibility to add specific test file _setup
use readlink in helper if realpath is not working as expected

plugins/python-build/test/installer.bats

@@ -3,7 +3,7 @@
 load test_helper
 
 @test "installs python-build into PREFIX" {
-  cd "$TMP"
+  cd "$BATS_TEST_TMPDIR"
   PREFIX="${PWD}/usr" run "${BATS_TEST_DIRNAME}/../install.sh"
   assert_success ""
 
@@ -18,7 +18,7 @@ load test_helper
 }
 
 @test "build definitions don't have the executable bit" {
-  cd "$TMP"
+  cd "$BATS_TEST_TMPDIR"
   PREFIX="${PWD}/usr" run "${BATS_TEST_DIRNAME}/../install.sh"
   assert_success ""
 
@@ -30,7 +30,7 @@ OUT
 }
 
 @test "overwrites old installation" {
-  cd "$TMP"
+  cd "$BATS_TEST_TMPDIR"
   mkdir -p bin share/python-build
   touch bin/python-build
   touch share/python-build/2.7.2
@@ -44,7 +44,7 @@ OUT
 }
 
 @test "unrelated files are untouched" {
-  cd "$TMP"
+  cd "$BATS_TEST_TMPDIR"
   mkdir -p bin share/bananas
   chmod g-w bin
   touch bin/bananas