[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/Set Colors/nvm_echo_with_colors

@@ -11,6 +11,7 @@ cleanup() {
   echo "Tested nvm_echo_with_colors"
 }
 
+: nvm.sh
 \. ../../../nvm.sh
 
 OUTPUT="$(nvm_echo_with_colors "\033[0;36mCyan-colored text")"

test/fast/Set Colors/nvm_err_with_colors

@@ -8,6 +8,7 @@ cleanup() {
   echo "Tested nvm_err_with_colors"
 }
 
+: nvm.sh
 \. ../../../nvm.sh
 
 set +ex

test/fast/Set Colors/nvm_print_default_alias calls nvm_get_colors

@@ -1,5 +1,6 @@
 #!/bin/sh
 
+: nvm.sh
 \. ../../../nvm.sh
 
 die () {

test/fast/Set Colors/nvm_print_versions calls nvm_get_colors

@@ -1,5 +1,6 @@
 #!/bin/sh
 
+: nvm.sh
 \. ../../../nvm.sh
 
 #set -e #nvm use system returns 127 and No system set message