$ cat all_tests.txt
test1
test2
test3
test4
test5
test6

$ cat completed_tests.txt
test1 Passed
test3 Failed
test5 Passed
test6 Passed

sort all_tests.txt completed_tests.txt

test1
test1 Passed
test2
test3
test3 Failed
test4
test5
test5 Passed
test6
test6 Passed

test1 Passed
test2
test3 Failed
test4
test5 Passed
test6 Passed

$ join -a 1 all_test.txt completed_test.txt
test1 Passed
test2
test3 Failed
test4
test5 Passed
test6 Passed

join -a 1  <(sort all_tests.txt) <(sort completed_tests.txt )

$ awk 'NR==FNR{a[$1]=$2; next}{print $1,a[$1]}' completed_tests.txt all_tests.txt
test1 Passed
test2
test3 Failed
test4
test5 Passed
test6 Passed

$ perl -lane '$t+=1; $h{$F[0]}=$F[1] if $.==$t; print $F[0]," ",$h{$F[0]} if $t!=$.;$.=0 if eof' completed_tests.txt all_tests.txt
test1 Passed
test2
test3 Failed
test4
test5 Passed
test6 Passed