[Tests] set $_ before sourcing nvm.sh in fast tests

nvm.sh uses `NVM_SCRIPT_SOURCE="$_"` to detect its source location.
Adding `: nvm.sh` before each source line ensures `$_` is set correctly, preventing breakage when the previous command (e.g., `set -ex`) overwrites it.

test/fast/Running 'nvm use x' should create and change the 'current' symlink

@@ -3,6 +3,7 @@
 set -ex
 
 export NVM_SYMLINK_CURRENT=true
+: nvm.sh
 \. ../../nvm.sh
 \. ../common.sh