Make libsecp256k1 tests optional

They are expensive and slow, especially on a slow hardware like
Raspberry Pi.

install.sh

@@ -280,9 +280,13 @@ libsecp256k1_build()
         --enable-benchmark=no \
         MAKE=$make
     $make
+    if [[ $use_secp_check == '1' ]]; then
         if ! $make check; then
             return 1
         fi
+    else
+        echo "Skipping libsecp256k1 tests."
+    fi
 }
 
 libsecp256k1_install()
@@ -371,6 +375,9 @@ parse_flags ()
             --develop)
                 develop_build='1'
                 ;;
+            --disable-secp-check)
+                use_secp_check='0'
+                ;;
             -p|--python)
                 if [[ "$2" ]]; then
                     python="$2"
@@ -403,6 +410,7 @@ Usage: "${0}" [options]
 Options:
 
 --develop               code remains editable in place (currently always enabled)
+--disable-secp-check    do not run libsecp256k1 tests (default is to run them)
 --python, -p            python version (only python3 versions are supported)
 --with-qt               build the Qt GUI
 --without-qt            don't build the Qt GUI
@@ -469,6 +477,7 @@ main ()
     develop_build=''
     no_gpg_validation=''
     python='python3'
+    use_secp_check='1'
     with_qt=''
     reinstall='false'
     if ! parse_flags ${@}; then