tests: Fix fate-run.sh to handle busybox-w32 absolute paths

Busybox-w32 uses regular Windows style paths with drive letters,
but with forward slashes; thus an absolute path starts with "c:/".

Make the target_path() function in fate-run.sh (which converts a
potentially relative path to an absolute one, under the target_path
prefix) handle this case.

With this in place, running fate tests almost works in
busybox-w32 - only one issue remains. A patch [1] has been sent to
upstream busybox for fixing that issue (which also is present if
running fate tests on busybox on Linux), but it hasn't been
responded to yet.

[1] https://lists.busybox.net/pipermail/busybox/2025-December/091851.html

tests/fate-run.sh

@@ -43,8 +43,17 @@ esac
 
 
 target_path(){
-    test ${1} = ${1#/} && p=${target_path}/
-    echo ${p}${1}
+    case ${1} in
+    [a-zA-Z]:/*)
+        echo ${1}
+        ;;
+    /*)
+        echo ${1}
+        ;;
+    *)
+        echo ${target_path}/${1}
+        ;;
+    esac
 }
 
 # $1=value1, $2=value2, $3=threshold