[fx][run-test] Don't strip = from forwarded test args

fx-standard-switches strips = from arguments past --, which should be
left intact and passed to the test invocation. Instead, filter the
arguments directly from the run-test arguments.

CF-324
Test: (pending local changes component-izing Bluetooth tests)
fx run-test bluetooth_tests -t bt-host-unittests -- --gtest_filter=\*

Change-Id: I235d5d18f4b23a5083c38dc404a8b4501ddd451d
diff --git a/devshell/run-test b/devshell/run-test
index 1d9971a..4f1649c 100755
--- a/devshell/run-test
+++ b/devshell/run-test
@@ -28,13 +28,14 @@
 
 
 function main {
+  all_args="$@"
+  test_args="${all_args#* -- }"
 
   fx-standard-switches "$@"
   set -- "${FX_ARGV[@]}"
 
   test=""
   target=""
-  test_args=""
 
   while (($#)); do
     case $1 in
@@ -48,8 +49,6 @@
         test="$1"
         ;;
       --)
-        shift  # value
-        test_args="$@"
         break
         ;;
       *)