run-tests: fix CLI

Restore the original behavior that was accidentally changed:
When no args are given, run the basic test suite.
Otherwise, run the given command with default scenario 'default'.
Previously, `run-tests.sh build` ran the basic test suite instead of
building the default scenario.

test/run-tests.sh

@@ -185,20 +185,22 @@ all() {
     scenario=netnsRegtest buildTest "$@"
 }
 
+# An alias for buildTest
 build() {
-    if [[ $scenario ]]; then
     buildTest "$@"
-    else
-        basic "$@"
-    fi
 }
 
-command="${1:-build}"
-shift || true
-if [[ $command != build  ]]; then
-    : ${scenario:=default}
-fi
+if [[ $# > 0 && $1 != -* ]]; then
+    # An explicit command was provided
+    command=$1
+    shift
     if [[ $command == eval ]]; then
         command=evalTest
     fi
+    : ${scenario:=default}
+elif [[ $scenario ]]; then
+    command=buildTest
+else
+    command=basic
+fi
 $command "$@"