Fix integration-tests issue with npm >= 7

Fixes the following error when running integration-tests:
```
npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR!
npm ERR! While resolving: webpack-test@1.0.0
npm ERR! Found: handlebars@5.0.0-alpha.1
npm ERR! node_modules/handlebars
npm ERR!   dev handlebars@"file:../../.." from the root project
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! peer handlebars@">= 1.3.0 < 5" from handlebars-loader@1.7.1
npm ERR! node_modules/handlebars-loader
npm ERR!   dev handlebars-loader@"^1.7.1" from the root project
npm ERR!
```

tests/integration/webpack-babel-test/test.sh

@@ -4,7 +4,7 @@ set -e
 
 # Cleanup: package-lock and "npm ci" is not working with local dependencies
 rm dist package-lock.json -rf
-npm install
+npm install --legacy-peer-deps
 npm run build
 
 for i in dist/*-test.js ; do

tests/integration/webpack-test/test.sh

@@ -4,7 +4,7 @@ set -e
 
 # Cleanup: package-lock and "npm ci" is not working with local dependencies
 rm dist package-lock.json -rf
-npm install
+npm install --legacy-peer-deps
 npm run build
 
 for i in dist/*-test.js ; do