tests: Quote make variables in TESTS_ENVIRONMENT

tests: Quote make variables in TESTS_ENVIRONMENT

Commit eb922a1b8f3a ("tests: use ${CC} instead of 'gcc' in tests")
exports ${CC} into the test environment, but doesn't quote the
value for the assignment. That doesn't work properly if the value
contains whitespace. In a multilib/biarch environment however, it's
common to set CC="gcc -m32" or similar. That causes tests to print
error messages: "/bin/sh: line 2: -m32: command not found".

Fix that by adding quotes around all make variables (not just $CC)
used in setting up TESTS_ENVIRONMENT.

Signed-off-by: Alexander Miller <alex.miller@gmx.de>