*: refactor methods to support mocking

Additionally, write some tests to ensure the driver method
`startBlacksmithBuilder` handles all exceptions correctly in
both nofallback=true and nofallback=false configurations.

jest.config.ts

@@ -1,30 +1,20 @@
-import fs from 'fs';
-import os from 'os';
-import path from 'path';
+import type {Config} from '@jest/types';
 
-const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'docker-build-push-action-'));
-
-process.env = Object.assign({}, process.env, {
-  TEMP: tmpDir,
-  GITHUB_REPOSITORY: 'docker/build-push-action',
-  RUNNER_TEMP: path.join(tmpDir, 'runner-temp'),
-  RUNNER_TOOL_CACHE: path.join(tmpDir, 'runner-tool-cache')
-}) as {
-  [key: string]: string;
-};
-
-module.exports = {
-  clearMocks: false,
-  testEnvironment: 'node',
+const config: Config.InitialOptions = {
+  clearMocks: true,
   moduleFileExtensions: ['js', 'ts'],
+  testEnvironment: 'node',
   testMatch: ['**/*.test.ts'],
   transform: {
     '^.+\\.ts$': 'ts-jest'
   },
-  moduleNameMapper: {
-    '^csv-parse/sync': '<rootDir>/node_modules/csv-parse/dist/cjs/sync.cjs'
-  },
-  collectCoverageFrom: ['src/**/{!(main.ts),}.ts'],
-  coveragePathIgnorePatterns: ['lib/', 'node_modules/', '__mocks__/', '__tests__/'],
-  verbose: true
+  verbose: true,
+  collectCoverage: true,
+  collectCoverageFrom: [
+    'src/**/*.ts',
+    '!src/**/*.d.ts',
+    '!src/**/__tests__/**'
+  ]
 };
+
+export default config;